Which among the following is the strongest Bronsted base?

Brønsted basic strength depends on the availability of the nitrogen lone pair for accepting: $$\mathrm{H^+}$$ 

Basicity is mainly influenced by: $$\mathrm{+I\ effect}$$ and  $$\mathrm{Steric\ Hindrance}$$ 

In: $$\mathrm{N(CH_2CH_3)_3}$$ (triethylamine), three ethyl groups increase electron density on nitrogen through: $$\mathrm{+I\ effect}$$

However, the bulky ethyl groups create significant steric hindrance around nitrogen, reducing protonation and solvation.

Hence, its basic strength decreases.

In:$$\mathrm{HN(CH_2CH_3)_2}$$ (diethylamine), two ethyl groups provide strong: $$\mathrm{+I\ effect}$$ with much lower steric hindrance.

Thus, secondary amines are generally stronger bases than ordinary tertiary amines in aqueous medium.

In: $$\mathrm{1\text{-}Methyl\text{-}3\text{-}pyrroline}$$ the adjacent: $$\mathrm{sp^2}$$ hybridised carbon atoms exert a slight: $$\mathrm{-I\ effect}$$

which decreases electron density on nitrogen. Hence, it is less basic than saturated aliphatic amines.

In: $$\mathrm{Quinuclidine}$$ the nitrogen lone pair is highly available. The bicyclic cage structure provides strong $$\mathrm{+I\ effect}$$ while preventing steric crowding around the lone pair.

Thus, protonation occurs very easily and the conjugate acid formed is highly stable.

Therefore, the strongest Brønsted base is:

$$\mathrm{Quinuclidine}$$

Correct option:

$$\mathrm{D}$$