Hint: Gases are the state of matter whose particles are very weakly attached to the other particles of the gas molecule. Since they are very weakly attached to each other they move in all directions. Light gas: The gas which has very less mass. So they move very freely.
Complete step by step answer: First of all we will discuss gases and the types of gases on the basis of their mass. Gases: The state of matter whose particles are very weakly attached to the other particles of the gas molecule, known as gases. Since they are very weakly attached to each other they move in all directions. Based on the weight of the gas, gases are divided into two categories: (i)Light gas and (ii) heavy gas Light gas: The gases which have less mass are known as light gas. For example: Hydrogen, helium. Heavy gas: The gases which have high mass are known as heavy gas. For example: Radon. Radon is a noble gas. Noble gases are those gases which generally do not react with other elements. They react with other elements or compounds but only at certain conditions. In the periodic table nobles gases are eighteen group elements which includes helium, neon, argon, krypton, xenon, radon. They have completely filled the outermost orbit so they generally do not react with other elements. And they are called as noble because they are stable in nature. And not react with others. Lightest gas is hydrogen as it has molecular mass of $2units$.Hence option C is correct. Helium is also a light gas but it is heavier than that of hydrogen. So the lightest gas is hydrogen.
So, the correct answer is Option C .
Note: Hydrogen and helium both are light gas as both have less mass. The mass of hydrogen is $2$ and mass of helium is $4$. As helium is light gas hence the balloons are filled with helium. And due to less mass they can fly easily.