Final answer:
Helium is the unreactive, gaseous element that is a product of the nuclear reaction of hydrogen atoms. It is the second most abundant element in the universe, but rare on Earth. Helium is used in blimps because of its low density.
Step-by-step explanation:
The unreactive, gaseous element that is a product of the nuclear reaction of hydrogen atoms and the second most abundant element in the universe, but rare on Earth, is helium (option a).
Helium is a noble gas from Group 18, which includes helium, neon, argon, krypton, xenon, and radon. Noble gases are called monatomic gases because they are composed of only single atoms and are unreactive at room temperature and pressure.
Helium's low density makes it suitable for use in blimps. It is often used to fill the gas chambers of blimps and balloons, as it provides buoyancy without causing flammability risks.