The word that best completes the statement is "son." Here's why:
1. In Spanish, "son" is the third-person plural form of the verb "ser," which means "to be." This verb is used to describe inherent or permanent characteristics of people or things.
2. In the given statement, the subject "Ellos" means "They" in English, indicating a plural third-person subject. Since the verb needs to agree with the subject in number, "son" is the correct form to use with "Ellos."
3. The word "católicos" is an adjective that describes the characteristic of the subject "Ellos." In this case, it means "They are Catholics."
Therefore, the complete statement would be: "Ellos son católicos." (They are Catholics.)
