The correct option is A Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
Mercury batteries using mercury oxide cathode have a very flat discharge curve, holding constant 1.35 V (open circuit) voltage until about the last 5% of their lifetime, when their voltage drops rapidly. The voltage remains within 1% for several years at light load, and over a wide temperature range, making mercury batteries useful as a reference voltage in electronic instruments.