As shown in the picture, when a hydrogen balloon with a slogan is exposed to the sun, its temperature increases and its volume expands. What remains unchanged is (　) A. hydrogen gas in balloon

A. Since the temperature of the balloon increases at this time, its internal energy increases, so it is wrong;

B. Since the total mass of hydrogen in the balloon remains unchanged, its volume becomes Large, so according to the density calculation formula, it can be seen that the density of hydrogen in the balloon decreases at this time, so it is wrong;

C. The density of the outside air remains unchanged, and the volume of the balloon becomes larger at this time, that is, it is displaced The volume of the gas becomes larger, so according to Archimedes' principle, the buoyancy force at this time becomes larger, so it is wrong;

D. At this time, the mass of the balloon and the internal hydrogen remains unchanged, so according to G =mg It can be seen that the gravity at this time does not change, so it is correct;

So choose D.