Sentence improvement (subject–verb agreement): match “what … is/are” with the true subject Original sentence: "What does agonise me most is not this criticism, but the trivial reason behind it." Choose the option that corrects the finite verb form.

Concept
In cleft-like structures, the verb agrees with the subject complement: “What … agonises me most is …”. The true subject is the what-clause treated as singular; the main verb must be third-person singular: agonises.

Option analysis
agonises me most — correct finite verb form and order. ✅most agonising me, agonising me most — participial fragments, not finite clauses.I most agonised — wrong subject and meaning.No correction required — original auxiliary “does” is unnecessary and ungrammatical here.

Final Answer
Correct core clause: “What agonises me most is not this criticism …”.