Guessing A as the closest correct answer, but in reality I would say it would increase and then remain constant.
Due to temp change increase below the tropopause maintaining a constant Mach until cross over altitude say "27000ft" would require an increase in downward pitch to maintain a constant CAS/IAS as dynamic pressure increases therefore drag increases. Once below cross over altitude to keep a constant CAS/IAS there maybe a small reduction in the descent pitch attitude required to achieve a decrease in TAS, with throttles closed.