a. From 52 total cards, you're drawing 4:

b. There are 13 hearts in the deck, and you want all 4 to be drawn from that pool. From the remaining 39, you are drawing nothing:

c. Similar to (b), but there are only 4 aces in the deck, and you don't care about the remaining 48 cards:

d. There are 13 ranks (ace, 2, 3, ..., 10, jack, queen, king) from which you want to draw 2. For each given rank, there are 4 possible suits, and a two of a kind consists of drawing 2 cards of the same rank, regardless of suit. Once you have two pairs, you ignore the remaining.

That is, there are
ways of getting two pairs if the two ranks are fixed. Then you multiply this by
, the number of ways you can get two different ranks.