Short-long day plants flower after:
Correct option is B, Short days followed by long days. In PHOTOPERIODISM problems like this, the accurate statement is Short days followed by long days; the others are common mix-ups. Not these: A) Long days followed by short days; C) Only short days; D) Only long days. Tissue transport questions usually turn on xylem vs phloem, living vs dead cells, or source vs sink.
Ref: Best CSIR NET Plant Physiology books: Master Unit 6 with Taiz & Zeiger and Salisbury & Ross. Crack Part C experimental questions with top textbooks.