Here, we are just adding up the numbers and then summing up the digits of the resultant number.

$7 \bigoplus 9 \bigoplus 10 = 7 + 9 + 10 = 26 = 2 + 6 = 8$ $9 \bigoplus 11 \bigoplus 30 = 9 + 11 + 30 = 50 = 5 + 0 = 5$ $11 \bigoplus 17 \bigoplus 21 = 11 + 17 + 21 = 49 = 4 + 9 = 13$

So, $23 \bigoplus 4 \bigoplus 15 = 23 + 4 + 15 = 42 = 4 + 2 = 6$

Hence, option (A) is correct.