Vasa recta are associated mainly with:
Juxtamedullary nephrons (B) is the right choice. This sits at the heart of Excretory System: the wording points straight to what is happening in the tissue or organ system. A useful study habit is to ask what the structure or process is for: the functional job described in the stem is exactly what Juxtamedullary nephrons does in Excretory System. That reasoning supports Juxtamedullary nephrons. One solid sentence about why this works is enough for recall — you do not need a long essay for every MCQ. When you practise, cover the choices first, write Juxtamedullary nephrons from memory, then reveal the letter — that habit builds real recall for Excretory System.
Ref: Animal physiology is Unit 7 of the CSIR NET Life Science Syllabus, covering core body systems. It accounts for roughly 10% of the total marks in Sections B and C.