Answer:
The answer is C. Participants nos. 6-10 were called out.
Explanation:
Nos. is the abbreviated version of numbers.
No. Is the abbreviated version of number.
Since more than one number is written (i.e 6-10), nos. will be used.
Then, because of subject-verb agreement in which the subject and verb should be either both singular or both plural, the word participants will be used with were.
Combining all these together will confirm C as the correct answer.