Double mutants of SWEET11 and SWEET12 show:
Sugar accumulation in source leaves (C) is correct here. This is core PHLOEM TRANSLOCATION: once you know the definition or pathway step, Sugar accumulation in source leaves is the clear fit. The wrong ones are A) Normal growth; B) Increased translocation; D) Enhanced sink strength. If the topic is about gradients or potentials, water/solutes move from higher to lower of the relevant quantity.
Ref: Best CSIR NET Plant Physiology books: Master Unit 6 with Taiz & Zeiger and Salisbury & Ross. Crack Part C experimental questions with top textbooks.