Which instrument discrepancies would require replacement of the instrument?

The scenario where the case is leaking, the glass is cracked, it will not zero out, and it is fogged indicates multiple severe issues with the instrument that compromise its functionality and reliability.

A leaking case suggests that the internal components are at risk of contamination from moisture or other environmental elements, which could lead to further degradation of the instrument's accuracy and integrity over time. A cracked glass can hinder visibility and may also expose sensitive internal components to dust and moisture. If the device will not zero out, it signifies a fundamental problem with the calibration or mechanical function of the instrument, rendering it unusable for precise measurements. The presence of fogging also indicates potential moisture accumulation inside the instrument, further impairing visibility and, possibly, performance.

Given these issues, it is clear that replacement is necessary to ensure that the instrument functions properly and delivers accurate results. Addressing all these concerns with repairs might not restore the instrument to the needed standards for reliability and performance, making replacement the best course of action.