The correct answer should be A. Whomever.
This is because this can be paraphrased as "the instructor has picked him" not "the instructor has picked he". Him corresponds to whomever while he corresponds to whoever.
Answer:
C.) a summary of the main points of comparison and contrast
Answer A makes sense for me
the other sentence is B therefore
because therefore means so
my teacher was very ill therefore (so) a substitute was called to finish the day
