Answer:
The correct answer is B. Monotreme embryos develop in an egg.
Step-by-step explanation:
Monotremes are the mammals that, like the platypus, develop in eggs. Thus, this class of mammals is the only one with this condition, which is shared with reptiles and birds.
Monotremes are considered to be animals with a prehistoric development, so this characteristic of being developed in eggs is a condition that originates in the historical period in which these animals emerged.