Answer:
Option C only measures time, option C is the correct answer.
Explanation:
Here we need to find which tool would you use to measure how long it takes a toy car to go down a ramp.
Here the unknown is time.
In the given options let us find what they measures.
   a) Ruler - Measures distance
   b) Scale - Measures distance
   c) Stopwatch - Measures time
   d) Meter stick - Measures distance
So option C only measures time, option C is the correct answer.