The group of mammals that is the most similar to reptiles is b) monotreme.
Monotremes are unique among mammals because they lay eggs instead of giving birth to live young, which is a reproductive trait more commonly associated with reptiles. This characteristic places them closer to reptiles in terms of evolutionary lineage. In contrast, marsupials and placental mammals give live birth, which further separates them from reptilian traits. Primates, while an interesting group, are also more distantly related to reptiles due to their reproductive and physiological differences.