**Core Concept**
Autoclaving is a sterilization process that uses high-pressure steam to kill microorganisms. The process involves heating the steam to a temperature of at least 121°C (250°F) at 15 psi for a specified duration. An indicator is used to ensure that the autoclave reaches the required temperature and pressure.

**Why the Correct Answer is Right**
The correct indicator used in autoclave is a biological indicator that contains a spore-forming bacterium, such as Bacillus stearothermophilus. This bacterium is resistant to heat and is used as a control to ensure that the autoclave has reached the required temperature and pressure to kill all microorganisms. The indicator is typically placed in a pouch or a container and is exposed to the steam during the autoclaving process. After the cycle is complete, the indicator is removed and cultured to check for the presence of the bacterium.

**Why Each Wrong Option is Incorrect**
**Option A:** Gastric pH indicator is used to monitor the acidity of the stomach, not for autoclaving.
**Option B:** pH indicator is used to monitor the pH level of a solution, but it is not suitable for autoclaving.
**Option C:** Thermometer is used to measure temperature, but it does not provide a biological indication of sterilization.

**Clinical Pearl / High-Yield Fact**
It is essential to use a biological indicator in autoclaving to ensure that the process has reached the required temperature and pressure to kill all microorganisms.

**Correct Answer: D. Biological indicator (e.g. Bacillus stearothermophilus).**