Answer: The verb that fills correctly the blanket is:
Cuando mi mamá era pequeña no tenían teléfonos celulares, pero a la edad de treinta y cinco ella compró su primer celular.
Explanation:
You can use the verb "era" when the time was a lapse, how the childhood, in the same case, in the second blank "tenían" applies for a lapse of time, but, the last blank is another case, you must use "compró" because the action to buy a cellphone is in an instant.
