Although egg-laying is rare in mammals, it is common for other amniotes. The fact that monotremes, such as the platypus, lay eggs
A. suggests this is an ancestral trait in tetrapods, while internal gestation is derived
B. demonstrates that the shelled amniotic egg evolved multiple times.
C. helps the species support an ectothermic life style.
D. indicates that they are more closely related to reptiles than to other mammals

The fact that monotremes, such as the platypus, lay eggs "indicates that they are more closely related to reptiles than to other mammals".

Option: D

Explanation:

The explanation that unusual, egg-laying mammals still exist today might be because of their descendants have taken to the water. The oviparous mammals, the word itself suggest something unique — the monotremes, such as the platypus, understood as eccentric relatives of the majority of the mammals that carry young living.

Other traits, apart from laying eggs, make them look more like reptiles than our family. They have a reptilian gait with legs on the sides instead of under the body, and a single opening for urine, feces and sex instead of multiple outlets.