Final answer:
The correct form of the verb is "escribió".
Step-by-step explanation:
The correct form of the verb to complete the sentence "Juan escribió un cuento." is escribió. This is because the subject Juan is singular and in the third person, so the verb form should also be singular and in the third person. The other options, escribí, escribieron, and escribiste, are all incorrect because they don't match the subject correctly.
Learn more about Verb conjugation in Spanish