A comparator is an indirect (relative) measuring instrument. Unlike a scale or micrometer that reads an absolute dimension directly, a comparator does not give the actual size of the part on its own — its whole operating principle is to measure the small deviation of the workpiece from a known standard, which is why the correct answer is comparison with a known standard such as slip (gauge) blocks.

The working procedure makes this clear:

1. A stack of slip gauges equal to the nominal (basic) size of the part is built up and placed under the measuring plunger.
2. The comparator's dial or scale is then set to zero against this standard — this step establishes the reference.
3. The slip gauges are removed and the actual workpiece is inserted. The instrument now displays only the difference (plus or minus) between the part and the standard, which is amplified mechanically, optically, pneumatically or electrically for easy reading.

This is precisely why the other options are wrong. Limit gauges (go/no-go gauges) are a completely separate class of tool that only tell whether a part is within tolerance, giving no numerical deviation and no comparison-to-standard readout. An optical measuring system and an accurately calibrated scale may appear in the amplification/readout of certain comparators, but they describe the display mechanism, not the fundamental principle — a comparator of any type (mechanical, pneumatic, electrical) still works by being zeroed against, and reading deviation from, a known standard. Hence the functioning is based on comparison with a known standard, such as slip gauges.