Concentrated urine is produced mainly due to action of:
ADH (C) 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 ADH does in Excretory System. That reasoning supports ADH. 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 ADH 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.