Select the antonym of
to muster

Let's check the meanings of the terms:
to muster: Assemble (troops, feelings, support), collect, or gather together.

to convocate: Call together; assemble. (Synonym-ish)

to rally: Bring together forces again; recover. (Synonym)

to aggregate: Form or group into a class or cluster. (Synonym)

to estrange: Cause (someone) to be no longer close or affectionate to someone; alienate or separate. (Antonym in terms of bringing together vs dividing/separating)

Hence, to estrange functions best as the antonym since it involves separating or isolating.