Absolute refractory period occurs because:
Go with B — Na+ channels remain inactivated. Within Action potential, that statement lines up with the standard definition and the usual exam wording you’ll see. In plain terms, cause and effect line up with Na+ channels remain inactivated — that is the mechanism or reason the stem is pointing to. Holding that picture in mind makes Na+ channels remain inactivated feel natural rather than something you only memorise. Link the term to a real body example (organ, tissue, or ion flow) so the idea stays concrete under exam pressure. A short mental diagram helps — start from stimulus or structure, follow the pathway, and stop at the functional result described by Na+ channels remain inactivated.
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.