Which of the following instrument discrepancies could be corrected by an aviation mechanic?

The choice indicating that a red line is missing and mounting screws are loose is correct in identifying an issue that an aviation mechanic can address.

In aviation, instrumentation must meet specific standards for safety and operational efficiency. A missing red line, which typically indicates critical limitations or thresholds, is a significant concern because it can lead to pilot misunderstanding during critical phases of flight. Additionally, loose mounting screws can compromise the integrity and reliability of the instrument's function, which is crucial for accurate operation. Both components can be rectified by an aviation mechanic during routine maintenance or inspections, thus ensuring the instrument functions correctly and adheres to aviation safety regulations.

The other options involve problems that may require more extensive repairs or component replacements, which may not fall within the scope of standard corrections performed by mechanics. For instance, a case leaking would necessitate a deeper investigation into the instrument's integrity and possibly involve repairs or replacement of the unit itself, while a cracked and fogged glass often requires specialty repairs or replacement rather than simple adjustments.