Actually in this sentence they don't make a pair - in the sense of giving the same meaning to the sentence. "Involved" means difficult or complicated when it is used to discuss something passive like ideas, so that pairs with Choice A.

"Involved" can mean "engaged in a personal relationship" but only when it is attributed to beings such as humans or other animals and not when applied to thoughts. You can have a personal relationship to your thoughts but they cannot have a personal relationship with you. Your thoughts can be complicated, though, so that is what involved means in this sentence.

"Engaged" means "busy or occupied" so clearly someone could be "engaged in the business of buying and selling jewels" and that person can also be "involved in the jewelry business." But see that this sentence works because we have an intelligent being at the center of the action. A person is busy or occupied. This means that in a different sentence these two words would make synonymous sentences! But the sentence would have to be one that involved people or animals.

This is actually a common tactic on the GRE. They will give you certain words - such as "obsequious" - that could not really be applied to anything but humans and maybe some few types of animals. This is one way to eliminate an answer or two!
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Explanation

Because the second half of the sentence illustrates the idea that “structure mirrors thought,” any word that fills the blank must be similar in meaning to “convoluted.” The two words that are similar to “convoluted” are “complicated” and “involved” (Choices A and F), which produce sentences alike in meaning. “Fanciful,” while somewhat similar in meaning to “convoluted,” is not as similar to either “complicated” or “involved” as those words are to each other. The other answer choices are not similar in meaning to “convoluted,” and thus do not produce coherent sentences.
Thus the correct answer is complicated (Choice A) and involved (Choice F).
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Why Not Engaged & Involved, They Also Make a Pair

The key word here is the first one A misconception

So, what is believed is NOT actually true or not always true.

Our thoughts are convoluted and complicated, more often than not. A such, we look for parallelism between our line of thought and the construction of a sentence.

The more it is complicated or involved in its unfolding the better it reflects our complicated line of thoughts.

Not true.

Hope this helps.

regards
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A misconception frequently held by novice writers is that sentence structure mirrors thought: the more convoluted the structure, the more _________ the ideas.

The sentence begins by stating that a misconception frequently held by novice writers is that sentence structure mirrors thought. Then after the colon we have an elaboration of the same idea through an example - the more convoluted the structure, the more ____ the ideas. Since what comes after the colon, explains, elucidates or describes what is before it, the statement before the colon and the example after it share the same kind of relationship. So since sentence structure mirrors thought, the more convoluted the structure, the more convoluted the ideas.

From the word list, complicated and involved are the answer choices.
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A and F

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