ANS:D - both (a) and (b) above

1. Its vapor pressure is practically zero: Mercury has an extremely low vapor pressure at normal temperatures, which means it does not evaporate readily. This characteristic ensures that the pressure measured by the barometer is due primarily to atmospheric pressure and not influenced by vapor pressure from the liquid itself.
2. The height of the barometer will be less: Mercury is a dense liquid, which means it requires less height in the barometer to balance the atmospheric pressure compared to less dense liquids like water. This makes mercury a practical choice for constructing compact and sensitive barometers.