Answer: Mientras yo estuve en clase esta mañana, mi mamá me compró un regalo de cumpleaños.
Explanation: Here we need to find which sentence meets the rule of simultaneous actions.
This is that both actions need to be conjugated in the same verbal time.
In the first sentence, the actions are "estuve" and "compró", both are in "preterito perfecto", so both actions where simultaneous.
The second sentence has the actions "Estaba" and "compraba", both in "preterito imperfecto", so both of them are in the same verbal time, but not necessarily are simultaneous.
The third option has the actions "Estuve" and "compraba", here the actions are in different verbal times, so this can be discarded.
The only option that meets the rule is the first one.
