Each sentence needs to follow the structure: subject pronoun + verb in the present tense.

¿Que hacen Mary y Lisa?
¿Que hace Lola?
¿Que hace Carlos y Sam?

Mary y Lisa are singing together
Lola is studying
Carlos y Sam are on the phone together

To determine the subject pronoun and the corresponding verb in the present tense, you need to identify who or what is performing the action in each sentence.

In the sentence "¿Qué hacen Mary y Lisa?" (What are Mary and Lisa doing?), the subject is "Mary y Lisa" (Mary and Lisa). The subject pronoun for this is "ellas" (they). Therefore, the verb in the present tense should be "hacen" (do/are doing). So the complete sentence should be "Ellas hacen" (They are doing).

In the sentence "¿Qué hace Lola?" (What is Lola doing?), the subject is "Lola." The subject pronoun for this is "ella" (she). Therefore, the verb in the present tense should be "hace" (does/is doing). So the complete sentence should be "Ella hace" (She is doing).

In the sentence "¿Qué hacen Carlos y Sam?" (What are Carlos and Sam doing?), the subject is "Carlos y Sam" (Carlos and Sam). The subject pronoun for this is "ellos" (they). Therefore, the verb in the present tense should be "hacen" (do/are doing). So the complete sentence should be "Ellos hacen" (They are doing).

Putting it all together, the complete sentences would be:
- "Ellas hacen" (They are doing) for "¿Qué hacen Mary y Lisa?"
- "Ella hace" (She is doing) for "¿Qué hace Lola?"
- "Ellos hacen" (They are doing) for "¿Qué hacen Carlos y Sam?"

You didn't use subject pronouns.