The right answer is c. son
Mi primo y yo conocemos a los jugadores del Real Madrid. Ellos son españoles
son is the conjugation of the verb ser (to be) for the third-person plural in the simple present. The verb ser is used to talk about permanent or lasting attributes. In this way, this verb stands for descriptions, occupations, characteristics, time, origin and relationships. In this sentence, the verb ser stands for descriptions. This talk about the essential qualities that describe a person or things like names, nationalities, physical descriptions and religions.
