Consider the machine M: The language recognized by M is :

[answerhappygod]

a) {w ∈ {a, b}* / every a in w is followed by ex¬actly two b’s}
b) {w ∈ {a, b}* every a in w is followed by at least two b’}
c) {w ∈ {a, b}* w contains the substring ‘abb’}
d) {w ∈ {a, b}* w does not contain ‘aa’ as a substring}