This question concerns the basic requirements of a measuring instrument, specifically what most directly preserves accuracy. Accuracy is how closely a reading agrees with the true value of the measured quantity. Over time, instruments drift — component aging, temperature changes, mechanical wear, and supply variations introduce systematic (zero/offset and gain/scale) errors that push readings away from the true value.

The single most important action to counter this is to calibrate the instrument against a known, certified standard reference. This is the correct choice. Calibration:

• Compares the instrument's indicated output to a reference of established, higher accuracy.
• Detects and lets you correct systematic errors such as zero offset and gain/span error.
• Establishes traceability — an unbroken chain of comparisons linking the reading back to national or international standards (e.g. NPL/NIST), which is what makes a measurement trustworthy and comparable.

Why the other actions are wrong — each degrades rather than protects accuracy:

• Operating always at maximum frequency range: instruments have a specified usable bandwidth; forcing maximum frequency generally introduces frequency-response and loading errors rather than improving accuracy.
• Reducing the sensitivity: lower sensitivity means the instrument responds less to small changes in the input, coarsening resolution and making it harder to read the true value accurately.
• Setting the amplifier gain to zero: this effectively kills the signal path, so the instrument produces little or no meaningful output — the opposite of an accurate measurement.