Select the most appropriate ANTONYM of the highlighted word in the given…

An antonym is a word that expresses the opposite meaning of a given word on the same axis of meaning (intensity, quality, or effect), while matching its part of speech and register. To find it, first pin down the precise sense and connotation of the highlighted word, then look for the option that expresses the diametrically opposite quality on that same axis, not merely an unrelated or loosely contrasting idea.

Here, "inflammatory" (adjective) describes speech or language that arouses strong, angry, or violent feelings and provokes unrest, as shown by the sentence's own consequence, "causing uproar among the audience." The direct opposite of arousing agitation is producing calm: an option that means soothing or pacifying stands on the exact opposite end of the excite-versus-soothe axis.

Checking each remaining option against that same axis shows why it does not qualify as the opposite:

• Predictable concerns whether something can be foreseen in advance, an axis of certainty rather than emotional intensity, so it does not oppose "inflammatory".

• Lengthy concerns duration or length; a short speech can be just as inflammatory as a long one, so length is an unrelated axis.

• Boring concerns a lack of interest; it does not sit on the excite-versus-soothe axis, so it is not the semantic opposite of a speech that provokes anger.

Standard usage confirms this pairing: dictionaries and thesauri consistently list "calming" or "soothing" as antonyms of "inflammatory" in the sense of speech or rhetoric, so the option expressing a soothing, tension-reducing effect completes the opposite pair.