I'm confused about the actual question where the width of a slit gets changed. A diffraction grating and a double slit are not exactly the same thing, because with a diffraction grating there are many slits, as I remember. Where are you getting these problems?
I'm going to assume that you mean a double-slit setup, and that the "width of one slit is reduced to" .... what? well, if you reduce it until it is lower than the wavelength of the light going through the apparatus, then there is no transmission through the slit, and it becomes a single-slit experiment. If not, you can solve the problem rigorously using a method of fourier transformations found in most math for physicists textbooks.
B doesnt make any sense unless "replaced" is replaced with "displaced".