**Core Concept**
Hot air ovens are used for sterilization of heat-stable materials by exposing them to dry heat at a high temperature. This process is suitable for materials that are sensitive to moisture, such as powders, granules, and some types of equipment.

**Why the Correct Answer is Right**
The dry heat sterilization process using a hot air oven is effective for materials that are not damaged by high temperatures. The heat is evenly distributed throughout the oven, allowing for thorough sterilization of the contents. This process is commonly used for sterilizing glassware, metal instruments, and other heat-stable materials.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is a placeholder and does not provide a specific material that is not suitable for hot air oven sterilization.
**Option B:** This option is incorrect because it is a placeholder and does not provide a specific material that is not suitable for hot air oven sterilization.
**Option C:** This option is incorrect because it is a placeholder and does not provide a specific material that is not suitable for hot air oven sterilization.
**Option D:** This option is incorrect because it is a placeholder and does not provide a specific material that is not suitable for hot air oven sterilization.

**Clinical Pearl / High-Yield Fact**
Some materials that are not suitable for hot air oven sterilization include biological agents, chemicals, and materials that are sensitive to heat or moisture. It is essential to check the manufacturer's instructions for sterilization of specific materials.

**Correct Answer: D. Biological agents.