Let's analyze the given equations:

1. 7 ⊕9 ⊕ 10 = 8
2. 9 ⊕11 ⊕ 30 = 5
3. 11 ⊕17 ⊕ 21 = 13

Here, the symbol ⊕ represents any mathematical operation. To find the pattern and the operation represented by ⊕, let's analyse each equation.

We are adding up the numbers and then summing up the digits of the resultant number.

7 ⨁ 9 ⨁ 10  = 7 + 9 + 10   = 26 => 2 + 6 = 8

9 ⨁ 11 ⨁ 30   = 9 + 11 + 30  = 50 => 5 + 0 = 5

11 ⨁ 17 ⨁ 21 = 11 + 17 + 21   = 49 => 4 + 9 = 13

So, 23 ⨁ 4 ⨁ 15  = 23 + 4 + 15   = 42 => 4 + 2 = 6

Thus, the correct option is (a).