Final answer:
The verb form that correctly completes the sentence is 'desayunan'.
Step-by-step explanation:
The correct verb form to complete the sentence is desayunan. This is because the subject of the sentence is plural, indicated by the word 'leche con cereales', which is the breakfast food being referred to. In Spanish, plural subjects require the verb form that matches the third person plural pronoun, which is 'desayunan'.
Learn more about Verb forms in Spanish