When all as a word means 'everything', a singular verb is used; but when it means a collection of people or things, a plural verb is used.

Examples:

All has been set for the final match. (Here ALL means everything)

All have left the stadium. (Here ALL means a collection of people)

The subject "All we need" is treated as a singular entity because of the indefinite pronoun "all", and the verb should agree with this singular subject. Therefore, "is" (singular) is the appropriate choice. Options A and D use plural verbs ("are" and "were"), and option B uses the past tense singular verb "was," none of which correctly matches the singular subject.