Which instrument discrepancies would require replacement of the instrument?

The correct choice encompasses critical physical and functional conditions that directly affect the safety and reliability of the instrument. A missing red line can compromise the functionality of an instrument, as this line typically indicates critical operational limits, such as airspeed or engine temperature thresholds. Without these clear indicators, a pilot could misinterpret the data and inadvertently operate the aircraft outside safe parameters.

Additionally, a leaking case can allow moisture to enter and damage the internal components of the instrument. If the glass is cracked, it not only poses a risk of further damage but also compromises visibility and readability of the instrument readings. All these factors combined indicate that the instrument can no longer provide reliable or safe readings, thus necessitating its replacement.

In contrast, while the other options contain significant issues, they may not individually lead to an immediate requirement for replacement. Issues like a chipped case paint or loose screws could potentially be remedied with repair or maintenance rather than necessitating a full replacement of the instrument, and a total malfunction might also point toward repairable issues depending on the specific circumstances. However, the combination of a missing red line, a leaking case, and cracked glass in the correct choice clearly illustrates a scenario where the integrity and usability of the instrument are too compromised for continued operation.