The formula for conditional probability given in Part A of the tutorial is

P(E|F)

=

P(EF) P(F)

.

Multiplying both sides by P(F) gives

P(EF) = P(E|F)P(F),

which is choice A. That is not the only correct choice, however; if we switch E and F, we get

P(FE) = P(F|E)P(E)

P(EF) = P(F|E)P(E),

which is choice D. Therefore, Choices A and D are both correct.