Which compound, out of the following options, has the highest boiling point: methane, ethane, propane, or butane?

For alkane chains; the longer the chain , the higher the boiling point.

Methane ; lowest b.p.

Butane ; highest b.p

Methane exists as a gas at STP.

Butane exists as a gas at STP , but if the temperature is lowered slightly, then it will become a vapour/ liquid.