Step-by-step Solution:

The series is 6, 11, 21, 36, 56, ?

Differences between consecutive terms:

• 11 - 6 = 5
• 21 - 11 = 10
• 36 - 21 = 15
• 56 - 36 = 20

The differences increase by 5 each time, so the next difference is 20 + 5 = 25.

Thus, the sixth number is 56 + 25 = 81.

Answer: (C) 81