look at my assumption ,
we know that every BOTTOM PARSER is an DCFL ,that means STACK can be used.
now, push the non terminals into stack WHICH ARE ONLY derivable from the given grammer llike we do in PUSH DOWN AUTOMETA.
1) SINGLE "a" cannot be derived from the G, eliminate it .
2)cannot END with "a" and more over starting symbol doesnt have EPSILON production ,SO eliminate it
3) TWO "b"s cannot be derived from the grammer ,so eliminate it
4) THIS can be derived,now push the string into the STACK , this string can be satisfied by PDA by popping up of any number of a's when b is on the top of the stack
push aas into the stack by taking
S ->aS/b
but , SINGLE 'b' can also be derivable and accepted by PDA ,it can also be an ANSWER if it is in the given in options