Correct answer is (a) Parts Per Million
Easiest explanation: The correct full form of the PPM abbreviation is Parts per million. It is a way of measuring the concentration of a particular gas in the air, by using the ratio of it and comparing it to others. For example: 1000 PPM of LPG means that if you count 1 million gas molecules within a closed space, 1000 of them would be LPG molecules while the rest would be other gases.
