**Core Concept:** The ignition of a gaseous mixture like methane and air requires a certain minimum concentration of fuel and oxygen for sustained combustion to occur.

**Why the Correct Answer is Right:** To prevent an explosion, the methane content in the mixture must be significantly reduced. In this case, we are looking for the minimum reduction in methane content that would make an explosion impossible. The correct answer is C, as it indicates that the methane concentration should be less than 5%.

**Why Each Wrong Option is Incorrect:**

A. 10% methane: This option is too high, as it exceeds the threshold required for explosion to occur.
B. 15% methane: This option is still within the range needed for explosion, so it is incorrect as well.
D. It is difficult to determine a specific percentage for this question, but the explanation above applies to this option as well.

**Clinical Pearl:**

Understanding the minimum ignition concentration of flammable gases like methane is crucial for safety measures in settings where such gases are present, such as in industrial settings or homes with gas appliances.

**Correct Answer:** The correct answer is C, where methane concentration should be less than 5%. This ensures that an explosion due to ignition of methane and air is impossible.