Which among the following is the strongest Bronsted base?

Cyclic amines are more basic than open chain amines of same degree. Because open chain amines undergo umbrella inversion which decreases the basic nature. Hence, among the given compounds, quinuclidine is strongest base.

And also quinuclidine is more basic and more nucleophilic than triethylamine, because of lesser steric hindrance in quinuclidine than in triethylamine. Lesser steric hindrance in quinuclidine is due to all the alkyl groups are “tucked” back and Nitrogen is more exposed and can easily donate its lone pair electrons.