**Core Concept:** Sterilization is the process of destroying all forms of life, including bacteria, viruses, and fungi, on medical equipment to prevent infections during surgical procedures. Sterilization methods can be physical (e.g., heat, radiation) or chemical (e.g., chemical disinfectants).

**Why the Correct Answer is Right:** Operation theatres are sterilized using a combination of both physical and chemical methods to ensure complete elimination of microorganisms. In this case, the correct answer is **D**.

1. **Option A (March 2007):** This is not related to sterilization methods, but rather refers to a specific month and year, which does not contribute to the process of sterilization.
2. **Option B:** Heat sterilization (autoclaving) is a common method used in operation theatres. It involves heating the equipment to high temperatures (121°C) under high pressure, which destroys microorganisms.
3. **Option C:** Chemical disinfection is another method used in operation theatres. Common chemical agents include glutaraldehyde, formaldehyde, and peracetic acid. These chemicals inactivate microorganisms, but they are often used in combination with physical methods, as mentioned in Option D.
4. **Option D:** A combination of heat sterilization (autoclaving) and chemical disinfection is the correct answer because it ensures complete elimination of microorganisms. Heat sterilization kills bacteria, viruses, and fungi, while chemical disinfection targets resistant microorganisms and prevents the development of antibiotic resistance. Using both methods together provides a comprehensive sterilization process for operation theatres.

**Why Each Wrong Option is Incorrect:**

1. **Option A (March 2007):** This is unrelated to sterilization methods and does not contribute to the process of sterilizing operation theatres.
2. **Option B (Heat Sterilization):** While autoclaving is a common method, using it alone might not be sufficient to eliminate all types of microorganisms, particularly resistant ones. Combining it with chemical disinfection ensures comprehensive sterilization.
3. **Option C (Chemical Disinfection):** While chemical disinfection is effective against resistant microorganisms and prevents antibiotic resistance, it may not be sufficient to kill all types of microorganisms, particularly heat-resistant ones. Combining it with heat sterilization ensures comprehensive sterilization.

**Clinical Pearl:** Sterilization is a crucial step in preventing infections and maintaining patient safety during surgical procedures. A combination of heat sterilization and chemical disinfection ensures a comprehensive sterilization process for operation theatres.