The answer you are probably supposed to select is (3) - volume of the solvent be doubled.
V is typically expressed in litres.
So to halve the value of M, you need to double the value of V.
Technically, you should really say "overall volume of the solution be doubled" rather than "volume of the solvent be doubled". It is being assumed here that the solute has a negligible effect on the overall volume of the solution. For an ionic solid thats a good assumption, but it might not always be.........