Why will a balloon filled with helium float in air?

Question

Responses

Helium is a diatomic molecule.
Helium is a diatomic molecule.

Helium is less dense than air.
Helium is less dense than air.

Helium is as dense as air.
Helium is as dense as air.

Helium is denser than air.

Answer 1

Helium is less dense than air.