Select the most appropriate option to fill in blank no. 2.
"Superheroes often have backstories that 2) ________ how they have gained their powers..."

The relative pronoun "that" refers back to the antecedent noun "backstories", which is plural. Therefore, it requires a plural finite verb in the present tense to match the general descriptive nature of the text.
* "explain" is the base form / plural form of the verb, which fits correctly.
* Options A and D are grammatically incorrect constructions ("was explain", "had explain").