Let the distance travelled at 10 kmph be \( x \) km.
Then, the time taken to travel \( x \) km is:
\[
\frac{x}{10}.
\]
The remaining distance, travelled at 18 kmph, will be \( (80 - x) \) km. The time taken to travel this distance is:
\[
\frac{80 - x}{18}.
\]
The total time taken is 6 hours. Therefore, we have the equation:
\[
\frac{x}{10} + \frac{80 - x}{18} = 6.
\]
Multiplying through by 90 (the least common multiple of 10 and 18) to eliminate the denominators:
\[
9x + 5(80 - x) = 540.
\]
Expanding and solving for \( x \):
\[
9x + 400 - 5x = 540,
\]
\[
4x = 140,
\]
\[
x = 35.
\]
So, the person travelled 35 km at 10 kmph. The percentage of the total distance travelled at 10 kmph is:
\[
\frac{35}{80} \times 100 = 43.75%.
\]
Thus, the correct answer is (C) 43.75.