C. rant
He went into a long rant about the outcome of the election after it was mentioned on the news.
assert and fastidious are not nouns, which is what the sentence is asking for "a thing", so they are not the correct answers. As well they only mention one person in the sentence and quibble is an argument, which needs more than one person. Therefore it is not likely to be the correct answer. A rant is a long tirade by one person, it fits the best within the sentence.