Question

what are fire extinguishers designed to remove?
heat or oxygen
hydrogen
flammable gas

Explanation:

Fire extinguishers work by removing one or more elements of the fire - triangle (heat, oxygen, fuel). They can remove heat or oxygen to stop the fire. Hydrogen and flammable gas are fuels, not what fire extinguishers are primarily designed to "remove" in the sense of eliminating a fire - starting condition.

Answer:

heat or oxygen