Phototropin receptors are mainly localized on:
C fits best: Plasma membrane. Keep the Phototropin+All basics straight and Plasma membrane stands out as the precise answer. Avoid: A) Nucleus; B) Chloroplast envelope; D) Cell wall. Light and flowering items often hinge on which photoreceptor or day-length rule is in play.
Ref: Best CSIR NET Plant Physiology books: Master Unit 6 with Taiz & Zeiger and Salisbury & Ross. Crack Part C experimental questions with top textbooks.