Answer:
C. Look at school performances to see if the test accurately predicts performances of both short and tall people.
Step-by-step explanation:
In this case, the best way to prove if the test is valid or not, is to contrast it with another set of data, and a good set is school performance.
If the test does predict school performance for tall and short students, then, the test can proven correct. If the experiment results otherwise, the test could be discarded, or reevaluated.