In GBN, sequence numbers used are $n$ for the sender, and $1$ for the receiver.
Hence, total sequence numbers are $n + 1$.
In the question, number of bits for sequence numbers = $x$.
Total sequence numbers possible out of it = $2^x$
Reserve 1 number(not 1 bit) for the receiver, so we get $2^x-1$ numbers for the Sender.
None of the Options is correct!