Question opens some ambiguity: unclear whether the people have a choice of 6 of 7 floors to get out of (i.e. whether the ground floor is considered floor 0 or floor 1). No answer fits the 7 floor scenario, so I'll go with choice of 6 floors, in which case the answer should be B. I posted on a similar question last Thursday.
My method (under spolier to give other people a chance):
[spoiler]there are 4 people, each of which has a choice of 6 floors to leave. Break it down to step-by-step:
6 ways for the first AND
6 ways for the 2nd AND
6 ways for the 3rd AND
6 ways for the 4th.
Every AND denotes multiply, so the final answer is 6*6*6*6 - not an easy calculation to make, but the answer should end with a units digit of 6, which only B does. [/spoiler]
