a0a*
F

My WA output N (infinite)
I interpret this as (a)concat(0)concat(a*) = {a, aa, aaa, aaaa, ...}

This expression describes words of the form xyz, where x comes from the set {a}, y comes from the set {} (i.e. empty set), and z comes from the set {empty_word, a, aa, aaa, ...}. Since y must be an element of an empty set, there is no such word of the form xyz, so the answer is F.