These all are Lewis bases as N-atom in each of them has tendency to donate an electron pair. Piperidine (c) is the strongest base among these. In pyrrole (b) the lone pair of electron on N-atom is involved in delocalization of π electrons (aromatic sextet) hence, it has least tendency to donate electron pair. Aniline (d) and pyridine (a) are also weakly basic due to resonance.
On the other hand piperidine has no such delocalization and it is a secondary amine.
Hence, it is highly basic.