Correct answer:
Nosotros estamos escribiendo cartas.
This sentence asks for changing the sentence from the simple present to the present progressive. This last tense is used to talk about something that is happening now. We form the present progressive by using the verb estar plus a present participle. So the formula is estar + present participle. Therefore, estamos is the conjugation of the verb estar for the first person plural in the simple present and escribiendo is the present participle of escribir.