Which is the most basic?

Which is the most basic molecule? The correct answer is D (piperidine).

Piperidine: The nitrogen atom in piperidine is sp3 hybridized. This means the lone pair of electrons resides in an orbital with 25% s character. Orbitals with less s character are higher in energy and the electrons are held less tightly by the nucleus. This makes the lone pair more available for donation to a proton (H+), which is the definition of a base.

Pyridine: The nitrogen atom in pyridine is sp2 hybridized. The lone pair of electrons occupies an orbital with 33% s character. Higher s character means the electrons are held more closely and tightly to the nucleus, making them less available for protonation.