Contain at least 2b
option(A):All string must end with b.
option(B) :All string must end with a.
option(D) :All string must end with b.
But our condition is that it must contain at least 2b which may end with a or b.So a,b,d eliminated.
at least 2b i.e may start with a or b i.e (a+b)*
then our condition two bb and it may contain a in between so (a+b)*ba*b
and it may end with a or b that lead to (a+b)*ba*b(a+b)*
So Option C is answer.