The correct answer isAbecause the most common and direct inspection instrument used for acoating after applicationis thedry film thickness DFT gauge. In source inspection and quality surveillance, coating verification typically includes checking whether the applied paint or protective coating has achieved the specified thickness range. That is a fundamental acceptance point because coating that is too thin may fail prematurely, while coating that is too thick may crack, blister, or cure improperly. In practical inspection language, the option says "Dry film thickness DFT," but it clearly refers to theDFT measuring gauge/toolused by inspectors.
The other options are related but not the best answer. Adigital surface profile gaugeandsurface profile replica tapeare mainly used to evaluate thesurface profile of blasted steel before coating, not the final dry coating thickness itself. Anultrasonic thickness meteris generally used for base material wall thickness measurement, not routine paint-coating inspection. The API guide covers source inspection and surveillance of manufacturing activities and stresses verification against specifications, records, and inspection points for covered equipment.