Right answer is (d) Voltage doesn’t change
Explanation: The MQ2 Gas Sensor has two output pins, one is digital and one is analog. The digital pin can only vary between two voltage levels which corresponds to 2 Logic Levels, 1 and 0 or 5V and 0V respectively. So if a gas is present then the digital output becomes 5V but cannot detect any kind of change in concentration of the gas.