For the same type of polymer, what generally happens as the crystallinity increases?

Answer: A. Density and modulus increase, and transparency decreases

In crystalline regions the chains are packed regularly and densely, so higher crystallinity raises the density and strengthens interchain interactions, improving modulus, yield strength, heat resistance and chemical resistance. This is why high-density polyethylene (HDPE) is stiffer than low-density polyethylene (LDPE). On the other hand, crystalline and amorphous regions differ in refractive index, and spherulites and other features comparable to or larger than the wavelength of light scatter it, so transparency falls and the material turns milky. Gases also pass through crystalline regions with difficulty, so gas permeability decreases. A decrease in density is the opposite of what happens, and high transparency is characteristic of amorphous polymers such as PMMA and polycarbonate.