answer is option ( c ) aaa
so in B-->a.B,a a after , is lookahead
by deriving stings we have to check rightmost derivation
since vaid viable prefix for item B-->a.B,a will be 'y' in S==> yBx such that there is a RMD in which x should have starting symbol as ' a ' ( corresponds to lookahead symbol)
so according to given derivation-->
S==>BB==>aBB==>aBaB==>aaBaB==>aaaBaB==>aaaBab here y=aaa , B=B , x=ab and starting symbol of x is ' a ' .
which is given in option is also ( c ) aaa.
hence answer is option ( c ) aaa