We are given the sum:
\[
S = 15 C_6 + 14 C_6 + 13 C_6 + 12 C_6 + 11 C_6 + 10 C_6
\]
This can be simplified by factoring out \( C_6 \):
\[
S = C_6 \times (15 + 14 + 13 + 12 + 11 + 10)
\]
Now, calculating the sum of the numbers inside the parentheses:
\[
15 + 14 + 13 + 12 + 11 + 10 = 75
\]
So, the expression becomes:
\[
S = 75 C_6
\]
Next, we observe that the binomial coefficients in the options suggest a shift in the terms. We know that:
\[
C_7 = C_6 + C_6
\]
Thus, the correct simplification for the given sum is \( 16 C_7 - 10 C_7 \), which matches option (C).
Thus, the correct answer is option (C), \( 16 C_7 - 10 C_7 \).