The correct form of the verb "estar" (to be) in your options is:
C. ellos están
Here’s a brief explanation of why:
- A. ustedes estamos: This is incorrect because "ustedes" (you all) should be paired with "están" (you all are).
- B. nosotros están: This is incorrect because "nosotros" (we) should be paired with "estamos" (we are).
- C. ellos están: This is correct because "ellos" (they) is appropriately matched with "están" (they are).
So the correct answer is C. ellos están.