What a mechanical comparator is: A comparator does not measure an absolute dimension directly; instead it measures the small deviation of a workpiece from a known reference (a slip-gauge stack or master), which it magnifies so the difference can be read on a scale. A mechanical comparator (e.g. dial indicator, Johansson Mikrokator, or gear-and-rack/lever types) achieves this magnification purely through mechanical elements — gear trains, levers, pivots, springs, and twisted strips.

Why the correct answer is right: Because magnification is produced by moving mechanical parts in contact, those parts (gear teeth, pivot bearings, lever fulcrums, plunger guides) are subject to friction and wear during repeated use. As they wear, backlash and lost motion increase and the calibration drifts, so accuracy and repeatability degrade over time. This makes "mechanical wear can reduce accuracy over time" the genuine limitation, and it is why mechanical comparators need periodic re-calibration and servicing.

Why the other options are wrong:

• Cannot measure linear dimensions is false — measuring linear deviations is precisely their main function.
• Cannot measure small deviations is false — their high magnification is designed specifically to detect very small deviations (a fraction of a micrometre in fine instruments); detecting small deviations is their whole purpose.
• They require electricity is false and is in fact an advantage of mechanical comparators — being purely mechanical, they need no power supply, unlike electrical, pneumatic, or optical comparators.