Parasympathetic control of heart rate is mediated mainly via:
Vagus nerve (B) is the right choice. This sits at the heart of Cardiac Cycle and Regulation: 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 Vagus nerve does in Cardiac Cycle and Regulation. That reasoning supports Vagus nerve. 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 Vagus nerve from memory, then reveal the letter — that habit builds real recall for Cardiac Cycle and Regulation.
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.