Must

From Twitter:

Dr. Smith, an archaeologist, finds an ancient body in a tomb in Egypt. It is the body of a man. There is a knife in his chest, and the man is holding it. Dr. Smith thinks them man:

a. must kill himself.
b. must be kill himself.
c. must have killed himself.

— SEI 新宿英会話教室 (Shinjuku English Institute) (@sayinsei) November 30, 2019

The answer is C, because it makes use of the present perfect (It talks about the past leading up to the present);

> Option A refers to the future:
‘must’ + action as in: “I must go.”

>Option B makes no sense at all:
‘must’ + be requires an adjective or verbal, next, as in:
George must be sick.
I must be going.