What makes the yardstick test a direct measurement of reaction time?
a. the further the yardstick falls, the slower the reaction time.
b. yardsticks have numerical markings for length.
c. yardsticks are designed to measure reaction time.
d. all of the above

I believe the answer is: a. The further the yardstick falls, the slower the reaction time.

In yarstick test, we need two persons. Someone who hold +/- 50cm yardstick from above between the hand of another person with fingers spread.

The holder than release the yardstick and the other is required to catch the yardstick as fast as he/she can.

Since the holder must release the yardstick without any cue to the catcher, the result would be 100% dependent on the catcher's reaction time.

a. The further the yardstick falls, the slower the reaction time.