The correct option is A Void because second wife is forbidden under hindu marriage law.
Any agreement object and consideration of which is forbidden by law or opposed to public policy is void as per section 23 of the Act. In this case, X has entered into an agreement to marry another girl although he is already married and his wife is alive but here the agreement is based on an act that is forbidden under law i.e. marrying more than once with living first wife is forbidden under Hindu Marriage Act, 1955 and hence, this agreement is unlawful and void.