Compared to the buoyant force of the atmosphere on a 1-kilogram helium-filled balloon, the buoyant force of the atmosphere on a nearby 1-kilogram solid iron block is

It all depends on the SIZE of the balloon.

If the balloon is made of really tough rubber, and it holds the helium in the same volume as the solid iron block, then the buoyant force of the atmosphere is the same for both objects.

But if the balloon is just some flimsy stuff, and it lets the helium expand to a much bigger volume than the iron block, then the buoyant force on the balloon is greater than the buoyant force on the solid iron block.

In fact, it DOESN'T MATTER what's in the balloon and what's in the block.  It doesn't matter whether either one of them is solid, liquid, or gas, and it doesn't matter whether they have the same or different mass.  

Whichever one has greater VOLUME has a greater buoyant force of atmosphere acting on it.

i guess it’s a and b

the duration of a photographic flash is related to an rc time constant, which is 0.100 μs for a certain camera.