**Core Concept**
The sterilization of endoscope tubes is crucial to prevent the transmission of infections to patients. This process involves the use of a combination of physical and chemical methods to eliminate microorganisms, including bacteria, viruses, and fungi, from the endoscope's complex structure.

**Why the Correct Answer is Right**
The correct method of sterilizing endoscope tubes typically involves the use of ethylene oxide gas, which is a potent bactericide and virucide. Ethylene oxide gas penetrates the endoscope's complex structure, including its narrow channels and crevices, to effectively eliminate microorganisms. This method is particularly effective in sterilizing endoscopes that cannot be easily immersed in a liquid sterilizing agent.

**Why Each Wrong Option is Incorrect**
**Option A:** Glutaraldehyde is a high-level disinfectant, but it is not a sterilizing agent. It is effective against a wide range of microorganisms, but it cannot eliminate all spores and viruses.
**Option B:** Autoclaving is a method of sterilization, but it is not typically used for endoscope tubes due to their complex structure and the risk of damage to the delicate instruments.
**Option C:** Hydrogen peroxide is a disinfectant, but it is not a sterilizing agent. It is effective against a wide range of microorganisms, but it cannot eliminate all spores and viruses.

**Clinical Pearl / High-Yield Fact**
When sterilizing endoscope tubes, it is essential to follow strict protocols to ensure the effective elimination of microorganisms. This includes using the correct sterilizing agent, following the recommended exposure time, and verifying the sterilization process using appropriate quality control measures.

**Correct Answer: D. Ethylene oxide gas.**