C

Step 1: Understand what percentage error means. Percentage error is a way to express how much an estimated or measured value deviates from the true value, relative to the true value.
Step 2: The formula for percentage error is given by:
$$ ext{Percentage Error} = rac{ ext{Absolute Error}}{ ext{True Value}} imes 100 $$
Here, the absolute error itself may have units (like meters, seconds, etc.), but when it is divided by the true value, the units cancel out.
Step 3: Since the percentage error is essentially a ratio of two quantities with the same units, the result is unitless.
Conclusion: Therefore, the unit of percentage error is indeed unitless.
Therefore, the correct answer is C.