My unprofessional opinion is b would be the answer
the first toss would have two outcomes.
the second toss would have four outcomes
and the third toss would have 8 outcomes
imagine you toss a coin with two sides, and your tree grows to branches
then you toss again, and out of the branches your tree just grew. it grows two more each. now imagine tossing for the third time and your tree growing for a third time
