Answer:
C) estan escribiendo
Step-by-step explanation:
In order to complete the sentence, you have to find the right form of the verb estar in present tense and this corresponds to the verb to be in English. The verb is conjugated as follows:
Yo estoy
Tu estás
El/Ella está
Nosotros estamos
Ustedes están
Ellos están
Then, the sentence refer to Ana y Marco which is they and in Spanish is Ellos. According to this, the right answer is Ana y Marco están escribiendo sus nombres en la arena.