Final answer:
Among methane, ethane, propane, and butane, methane has the lowest boiling point at -161.5°C, due to its lower molecular mass and the corresponding lesser strength of London dispersion forces.
Step-by-step explanation:
When evaluating which of the following straight-chain alkanes has the lowest boiling point (methane, ethane, propane, or butane), we need to consider the intermolecular forces and molecular masses of these compounds. Alkanes are nonpolar molecules, thus the predominant intermolecular force they experience is the London dispersion force, which tends to increase with molecular mass.
Given the molecular masses and boiling points listed, methane (CH4) has the lowest molecular mass at 16 g/mol and the lowest boiling point at -161.5°C. Ethane (C2H6), with a molecular mass of 30 g/mol, has a boiling point of -88.6°C. Propane (C3H8) and butane (C4H10) have higher molecular masses of 44 g/mol and 58 g/mol, respectively, and correspondingly higher boiling points of -42.1°C for propane and -0.5°C for butane.
Thus, the answer is methane, as it has the lowest boiling point among the given alkanes.