The answer to the given question above would be the last option. The sentence that contains gender-neutral language is the following one: Recycling helps preserve the Earth for all humankind.
Humankind is an abstract noun which refers to all the humans in the world: children, women and men. On the contrary, sentence A, "the firemen loaded (...)" makes use of the noun "firemen" which takes "men" as a suffix and it only refers to the men. "Steardness" refers to women who take care of passengers in either a shift or an aircraft. Finally, the last sentence takes "chairman", so the same reasons as in the first sentence apply.