Answer: A. hace, forma
Cuando se hace ejercicio, se forma mejor cuerpo.
When exercise is done, a better body is formed.
Explanation: this is a passive reflexive sentence. In these sentences, the form precedes a third-person verb in the singular or plural, depending on whether the nominal element that appears next to the verb and which is its grammatical subject is singular or plural. Because it is a form of passive, this construction only occurs with transitive verbs: "Cuando se hace ejercicio". The direct object is "ejercicio". "Se forma mejor cuerpo". The direct object is "mejor cuerpo". Normally, the subject of these sentences denotes something: "exercise, better body", but it can also denote an indeterminate person.
