The correct option is A Mode of reproduction
An echidna is an oviparous mammal. It reproduces by laying eggs. It has body hair and feeds its young ones with milk. It is a terrestrial animal.
A frog is an oviparous animal that also reproduces by laying eggs. But neither does it has body hair, nor it feeds its young ones with milk. It is an amphibian, i.e., it can live both on land and also in water.
So the only point of similarity between an echidna and a frog is their mode of reproduction.