Explanation

It was difficult to tell whether his tirade in fact had force of fact means that it was difficult tell whether someone’s angry speech (fulmination) was actually true. It is hard to tell whether his speech is “motivated by malice” (that is, “malicious lies,” or aspersion) or “reportorial” information (muckraking “exposing corruption”) that should actually be listened to.

Note that calumny is similar to aspersion and cannot work in the third blank — while the second blank describes negative speech meant to cause harm to others, the third blank describes negative speech that exposes the truth.

Fulmination, aspersion, muckraking.
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It was difficult to tell whether his tirade in fact had force of fact, whether his forceful (i) _________ was mere (ii) _________, motivated by malice, or whether it ranked as reportorial (iii) _________ of which the committee ought take notice.

Since we are told that he was indulging in a tirade, the word for the first blank is fulmination. A tirade is a long, angry speech of criticism or accusation. A fulmination is an expression of vehement protest.

Now, two opposite choices are suggested for us between the second and third blank by the phrasal structure "whether....or". For the second blank, the phrase "was mere (ii) _________, motivated by malice" suggests that we need a word which is similar in nature to fulminations but of lesser intensity. Therefore, clearly aspersions is the best choice. The word for the third blank should be one that is the opposite of aspersions - a false or misleading charge meant to harm someone's reputation. Therefore, muckraking - the action of searching out and publicizing scandal about famous people - is the best choice.
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