A. You have 7 items, and are meant to chose 4 of them to form a number. This is a permutation yes? How would you solve a permutation where you have N items and need to choose K?
B. This is a "tricky" one that requires a quick bit of reasoning. For a number among the amount you found in part A to be odd, it should have the LAST digit as the 1, 3, 5 or 7. Correct? Let one of those numbers be our base. We are then left with 6 numbers and we need to choose 3 to pair with our odd number. Do you see where I am going with this?
C. The same process as B.
Let me know what you get.