The correct answer is: Hydrogen has a heat transfer co-efficient 2.5 times that of air.
Air is a mixture of gases, mostly nitrogen and oxygen. Hydrogen is a gas. The heat transfer coefficient of a gas is a measure of how well it can transfer heat. Hydrogen has a higher heat transfer coefficient than air because it is a lighter gas. Lighter gases can move more easily and transfer heat more quickly.
Option A: 1.5 is incorrect because hydrogen has a higher heat transfer coefficient than air.
Option B: 2.5 is correct because hydrogen has a heat transfer coefficient that is 2.5 times that of air.
Option C: 3.5 is incorrect because hydrogen has a heat transfer coefficient that is less than 3.5 times that of air.
Option D: 4.5 is incorrect because hydrogen has a heat transfer coefficient that is less than 4.5 times that of air.