The assertion in the question states that the main component of natural gas is butane. Natural gas is actually primarily composed of methane, with smaller amounts of other gases like ethane, propane, and butane. Therefore, the assertion (A) is false.

The reason given in the question is that butane is easily liquefied under pressure. This is true, as butane can be converted into a liquid state by applying pressure. Therefore, the reason (R) is true.

Since the assertion (A) is false and the reason (R) is true, option 4 ("A is false but R is true") is the correct answer.

It`s important to note that even though butane can be found in natural gas, it is not the main component. Methane is the primary gas in natural gas, making up a significant percentage of its composition.