**Core Concept**
Autoclaving is a process used for sterilization of equipment and materials using high-pressure steam, and it relies on the principle of achieving a temperature of at least 121°C under pressure to kill microorganisms.

**Why the Correct Answer is Right**
Autoclaving involves the use of a vacuum pump to remove air from the chamber, followed by the introduction of high-pressure steam. The steam is then heated to a temperature of 121°C under a pressure of 15 psi (pounds per square inch) to achieve the desired level of sterilization. This process is effective against a wide range of microorganisms, including bacteria, viruses, and spores.

**Why Each Wrong Option is Incorrect**
**Option A:** This is an incorrect temperature for autoclaving, as 90°C is not sufficient to achieve sterilization.
**Option B:** This is an incorrect pressure for autoclaving, as 5 psi is lower than the required 15 psi to achieve the desired level of sterilization.
**Option C:** This is an incorrect combination of temperature and pressure for autoclaving, as 121°C with 5 psi is not sufficient to achieve sterilization.

**Clinical Pearl / High-Yield Fact**
It is essential to verify that the autoclave is functioning correctly before using it for sterilization, as incorrect settings can lead to inadequate sterilization or even damage to equipment.

**Correct Answer: D. 121°C at 15 psi.**