Which among the following has lowest boiling point?

Boiling point comparisons for aldehydes rely primarily on molecular weight and intermolecular forces.
Increasing molecular weight enhances van der Waals interactions, raising boiling points.

Molecular weights correspond to carbon counts:

$\text{Pentanal (C}_5\text{H}_{10}\text{O)} > \text{Propanal (C}_3\text{H}_6\text{O)} > \text{Ethanal (C}_2\text{H}_4\text{O)} > \text{Methanal (CH}_2\text{O)}$

Methanal, with the smallest molecular mass and weakest intermolecular forces, exhibits the lowest boiling point.