A letter-number designator assigned to a geographic location✓Correct answer
A letter-number designator assigned to an azimuth and elevation
An instrument for locating faults in power amplifiers
An instrument for radio direction finding
Explanation
A grid locator, in the Maidenhead system, is a short letter and number designator such as FN31 that encodes a rectangle of latitude and longitude, so it names a place on Earth compactly. It describes a location, not a pointing direction, and it is not an instrument at all.