Part (a) is relatively straight forward.
There are 4 aces in a standard 52-card deck. So the probability that AN ace is picked is just 4/52.
Part (b):
Note that there are 4 suits in a standard 52-card deck. So picking hearts will be 13/52 or 1/4.
Part (c):
The probability condition OR means we need to add the two probabilities because we do not want them simultaneously. So, we end up getting Pr(ace) + Pr(heart).
From part (a), we found that picking an ace had a probability of 4/52, which is reduced down to 1/13.
From part (b), we found that picking a heart had a probability of 1/4.
So, the probability of an ace OR a heart is 1/13 + 1/4 = 4/52 + 13/52 = 17/52.