Ya escribí las cartas. las cartas están
a. escribidas b. escritos c. escritas

For this case what we should do is fill in the blank with the correct verb,

We note that the correct verb for this case is:

"Escribir"

To fill in the blank we must conjugate the verb in simple present.

We must conjugate the verb for the third person plural.

We have then that the complete sentence in Spanish is:

Ya escribí las cartas. Las cartas están escritas.

The translation of the sentence in English is:

I already wrote the letters. The letters are written.

c. escritas

Ya escribí las cartas. Las cartas están escritas.

answer: the answer is b. se quejan because it's in the plural form.