STRATEGY: Upon reading any GRE Multiple Choice question, we should always ask, Can I use the answer choices to my advantage?
In this case, we can easily test the answer choices.
From here, I'd typically give myself up to 20 seconds to identify a faster approach, but I can already see that I can immediately eliminate 4 of the answer choices.
Here's why:

Given: Four years ago, the man was 7 times as old as his son was at that time .
Check the other choices....
(A) 28 years. If the man's present age is 28, then 4 years ago, he was 24, which means his age can't be a multiple of 7. Eliminate.
(B) 32 years. If the man's present age is 32, then 4 years ago, he was 28, which is a multiple of 7. Keep.
(C) 40 years. If the man's present age is 40, then 4 years ago, he was 36, which means his age can't be a multiple of 7. Eliminate.
(D) 44 years. If the man's present age is 44, then 4 years ago, he was 40, which means his age can't be a multiple of 7. Eliminate.
(E) 48 years. If the man's present age is 48, then 4 years ago, he was 44, which means his age can't be a multiple of 7. Eliminate.

If we want to be safe, we can check out the choice B (32 years).
If the man's present age is 32, then 4 years ago, he was 28, which means his son was 4 years old 4 years ago (since we're told the man was seven times as old as his son at that time)
If the son was 4 years old 4 years ago, and then the son is presently 8 years old, and the man's present age (32) is four time's the son's age.

Answer: B