Potassium deficiency mainly affects:
Older leaves first (B) is correct here. This is core MINERAL Transport: once you know the definition or pathway step, Older leaves first is the clear fit. The wrong ones are A) Young leaves first; C) Roots only; D) Flowers only. 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.