What must the tank be when measured with a gauge or stick?

When measuring the contents of a tank using a gauge or stick, it is essential that the tank is always level. This is because a level tank ensures that the liquid inside is distributed evenly, allowing for an accurate measurement of the contents. If the tank is tilted, the liquid would shift towards one side, which could result in an incorrect reading. An accurate gauge or stick measurement requires the tank to be positioned such that the measurement reflects the true volume of the liquid across the entire tank's cross-section.

In contrast, options suggesting that the tank must be tilted, full, or empty do not support accurate measurement practices. A tilted tank leads to erroneous readings, while measuring a full or empty tank does not account for the need for consistency in measurement techniques, which is crucial for ensuring safety and compliance in various applications.