FOLLOW(C) will be terminals that can come just after C.
Here FOLLOW(C) = { FIRST(B), FOLLOW(S) }
I wrote FOLLOW(S) because B can be epsilon, thus making S ->aA / C, thus FOLLOW(S) is included in FOLLOW(C)
FIRST (B) will give you {b, FIRST(A)}.. and FIRST(A) is also b,
therefore FOLLOW(C)= {b,$}