Final answer:
The verb that correctly completes the sentence is 'conocen'.
Step-by-step explanation:
The correct verb to complete the sentence is conocen.
The subject Pedro y Marta is plural, so we need to use the plural form of the verb. In Spanish, the verb conocer means "to know", and in the present tense, for the subject ellos/ellas (they), we use the form conocen.
Therefore, the correct sentence would be: "Pedro y Marta conocen a mi hermano." This means "Pedro and Marta know my brother."
Learn more about Spanish verbs