Explanation

An overview of the passage suggests that the first sentence is relatively self-contained and that the blank is answerable without the succeeding sentences, where the topic shifts slightly. The colon after the first blank signals that what follows will define the word in the blank and will explain what danger Mill was concerned about. It says that without challenge, one’s opinions grow “weak and flabby” and therefore one becomes complacent, not tendentious or fractious. A quick reading of the next two sentences suggests that the topic will be another danger that Mill described, “the danger of partiality and incompleteness.” Free and open discussion needs to take place because each person’s opinion tends to “embrace only a portion of the truth” and others’ views are partially right, or never completely “erroneous.” The other choices for the second and third blanks deal with change, immediacy, or antithesis, none of which relate to the
second danger of “partiality” or “incompleteness.”
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Why is "antithetical" ruled out in the last blank? It means opposite and opinions opposed to one's own rarely turn out to be completely opposite, sharing some similarities that help in encompassing the entire truth, because one's opinion at a time can only "partially cover the truth".

Can someone explain?

What readers most commonly remember about John Stuart Mill’s classic exploration of the liberty of thought and discussion concerns the danger of (i)__________: in the absence of challenge, one’s opinions, even when they are correct, grow weak and flabby. Yet Mill had another reason for encouraging the liberty of thought and discussion: the danger of partiality and incompleteness. Since one’s opinions, even under the best circumstances, tend to (ii)__________, and because opinions opposed to one’s own rarely turn out to be completely (iii)__________, it is crucial to supplement one’s opinions with alternative points of view.




Blank (i) Blank (ii) Blank (iii)
A. tendentiousness D. embrace only a portion of the truth G. erroneous
B. complacency E. change over time H. antithetical
C. fractiousness F. focus on matters close at hand I. immutable


So, key words here:
i) the danger of () in the absence of challenge. What do we get without challenge? Not tendentiousness (testiness), or fractiousness (likely to become a fight). Complacency.
ii) additional reason: danger of partiality and incompleteness. One's opinions tend to embrace only "a portion of the truth".
iii) it's crucial to supplement one's opinions with alternative points of view. Why? Because opinions opposed to one's own rarely turn out to be completely "erroneous".
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Because antithetical to a view means completely going totally against it.

For instance, if one doesn't believe in climate change, then their views are totally against someone who is a climate activist. It is sort of like black and white case. Either you are for it or against it. There is no partial or incompleteness.

Erroneous things can be partially correct. For example, ignoring some key elements of facts and jumping on conclusion would be considered "erroneous", but not antithetical.

Hope it makes more sense.
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Its a support of the second blank. When one considers only a partial view of something and base their opinion/views on that then they're highly unlikely to consider it error prone. Hence, erroneous fits here. Antithetical just means incompatible or opposed to, which doesn't go with the sentence.

Hmm, I was going back and forth between D and F. I was going to choose D but the "truth" in the A/C was unsupported by the passage, there was no mention of "truth" only "liberty of thought and discussion". Also F, by focusing on what's closest to us, aren't we partial / incomplete? How to arrive at D with this chain of thought?

So, basically you do not know how to deal with between D and F ?
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Two reasons to choose D:

1) Since, Mill mentions partiality and incompleteness - it leads us to believe that the opinions are not complete or have only a portion of truth.
2) Since, supplementing one’s opinions with alternative points of view is crucial - it again emphasizes that the original opinion is not complete