When conducting scientific research, the of a measure refers to whether it really measures what researchers think it measures?

The correct answer is validity. 

Validity refers to the ability of a test or study to actually to measure what researchers think it measures or what it is designed to measure. For instance, if a test or study aims to measure a population sample's blood alcohol level, but ends up measuring blood sugar levels instead, it lacks validity, because it did not measure what it set out to measure.