You are holding four identical balloons each containing 10.0g of a different gas. The balloon containing which gas is the largest balloon?

Hydrogen, H_2

Explanation:

mass of each gas is 10.0 g

number of mole = mass / molar mass

number of moles is directly proportional to volume at constant temp and pressure

this implies that the volume is inversely proportional to molar mass. And Among all the gases in periodic table the molar mass of Hydrogen is the least.

molar mass of H2=2 g/mol

Since, H2 has minimum molar mass then for the same mass of the gases Hydrogen will have maximum volume.