We must not ___ faith in humanity.

d) lose

The complete sentence would be: "We must not lose faith in humanity."

Explanation: To complete this sentence, we need a base form of verb as there is a modal auxiliary verb (must) before the blank. "Lose" is the correct word here, and it means to no longer have something, in this case, faith in humanity. It's important to use the correct spelling in this context.

The other options are not correct for this context and here's why:

a) The word "Loose" is an adjective which means not tight or not firmly attached. It is a completely different word and not suitable in this context.

b) The word "Louse" is a singular form of "lice," which is a noun meaning a type of parasitic insect. It is unrelated to the idea of having or losing faith.

c) "Loss" is a noun that refers to the act of losing something or the state of not having something. It is not the verb form required in this sentence.