Answer: a. Validity
In statistics 'validity' measures if a test or an experiment accurately measures what was intended to be measured.
A test with a high validity will be highly consistent with the test's objective. When a test's results are valid the conclusions drawn from it will be solid and can be acted upon.
In the question, Jacob got the same answer each time he performed the experiment, so his experiment was reliable.
However, since the manager pointed out that the experiment did not measure what it was supposed to (association between his outcomes and buyer's behaviour), Jacob's test lacked validity.