Option 1: Adding a sulfur containing compound to LPG does not lower the cost of production. The cost of production is determined by the extraction and refining process of propane and butane, not by the addition of a sulfur containing compound.

Option 2: Adding a sulfur containing compound does not enhance the efficiency of LPG. The efficiency of LPG is primarily determined by the combustion process and the energy content of the propane and butane, not by the addition of sulfur.

Option 3: The correct answer. Adding a sulfur containing compound to LPG facilitates easy detection of leakage of the gas. Sulfur has a distinct odor, and adding it to LPG allows users to easily smell any gas leakage. This is important for safety reasons, as gas leaks can be hazardous.

Option 4: Adding a sulfur containing compound does not assist in liquefying hydrocarbons. LPG is a mixture of hydrocarbons, including propane and butane, which are already in a liquid state. The addition of sulfur does not play a role in the liquefaction process.

Therefore, the correct answer is option 3 - adding a sulfur containing compound to LPG facilitates easy detection of gas leakage.