The missing term is 81.
The sequence is a list of numbers where each number is the sum of the two numbers before it. So, the first number is 6, and the second number is 11, which is the sum of 6 and 5. The third number is 21, which is the sum of 11 and 10. The fourth number is 36, which is the sum of 21 and 15. The fifth number is 56, which is the sum of 36 and 20. The sixth number is 81, which is the sum of 56 and 25.
Option A, 42, is not the missing term because it is not the sum of the two numbers before it. Option B, 51, is not the missing term because it is not the sum of the two numbers before it. Option C, 81, is the missing term because it is the sum of the two numbers before it. Option D, 91, is not the missing term because it is not the sum of the two numbers before it.