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