Answer: It's indeed A) begin
Step-by-step explanation:
None of the other options could be used, even if the context were different. The verb "commence" has the following definition: to have or make a beginning. It's a synonym to the following: begin, start, open, embark, kick off, and launch, among others.
Thus, the sentence could be rewritten as: "All functions of your body not needed for the struggle about to begin are shut down."