Olvidamos el champú.

a.

la olvidamos

b.

los olvidamos

c.

lo olvidamos

In this problem, we need to change the original sentence for a sentence using a direct object pronoun. This type of pronouns replaces a direct object, that is a noun that directly receives the action of a verb in a sentence. In this sentence el champú receive the action of the verb olvidar (to forget). Since el champú is given in masculine form and matches the third person singular, we need to choose the direct object pronoun that matches the third person singular in masculine form, that is lo.