Which key morphological feature is used to classify organisms as mammals
a. the presence of fur
b. fertile offspring
c. mammary glands
d. two-legged upright posture

C. Mammary glands

Explanation:

The mammals have multiple different morphological features that distinguish them from the other animal groups, some are unique to them, some are unique as a combination. The morphological feature that is unique for the mammals and is most recognizable for them are the mammary glands. The mammary glands are actually the reason why this group of animals has the name it has. All three types of mammals, placentals, marsupials, and monotremes have these glands through which they feed their young.

Lancelet

Explanation:

This is because a Lancelet is a small elongated marine invertebrate that lacks all of the physiological and biological traits of a leopard.