Question

What extreme action did one Buddhist monk take to protest the government of Ngo Dinh Diem in South Vietnam?

Answer 1

They set themselves on fire.

Explanation:

This was a very extreme and terrible way to protest:
normally two monks would walk up to a busy road, crossing, square or in front of a governmental building and after few words exchanged one of the monks would sit down in the lotus position reaching an higher state of meditation/contemplation (I hope) while the other, after pouring flammable liquid on him, would lit the fire.

I saw some pictures and really the monk was not moving a muscle or reacting (even if the pain would have been horrible) and this led me to think that together with prayers, meditation and superior interior peace there must have been, probably, also a more mundane components of drug induced stupor (but this is my opinion only).