When you replace helium in a balloon with less-dense hydrogen, does the buoyant force on the balloon change if the balloon remains the same size?

No change

Explanation:

First, we hare to understand what we mean by buoyant force.

Archimedes Principle states that

"the buoyant force on a submerged object is equal to the weight of the fluid that is displaced by the object."

Hot air balloons rise into the air as a result of the density of the air inside the balloon is less dense i.e warmer air than the air outside the balloon i.e cooler air. This is basically how the balloons work. Now applying this to our question

Replacing Helium with less dense Hydrogen will make no difference to the buoyant force because the volume of the balloon did not change. The buoyant force depends on the weight of the displaced air, and not on the force causing the displacement.

i think the answer would be b