What type of novel primarily explores social customs and habits?

A novel that primarily explores social customs and habits is referred to as a novel of manners. This genre is characterized by its focus on the behaviors, traditions, and values of a specific social class or group, often highlighting the intricacies of social interactions and the impact of societal norms on individuals. Through detailed observations and critiques, the novel of manners aims to illuminate the ways in which these customs shape personal relationships and social dynamics.

In contrast, an epistolary novel is structured as a series of letters or documents, which allows for a unique perspective on the narrative but does not primarily focus on social customs. A sentimental novel aims to evoke strong emotional responses from readers through vivid portrayals of love and compassion, rather than the exploration of societal behaviors. Lastly, a pastoral novel depicts rural life and the idealization of nature, which may touch on social customs in a limited way but does not center on them as its main theme. Thus, the focus on social customs and habits distinguishes the novel of manners as the correct answer.