While there are a number of characteristics that vary between monotremes and marsupials, there is one characteristic that links them together. what is it?
a. they both have nipples for sucking
b. they both have a single opening for reproduction, excretion and defecation
c. they both gave birth to underdeveloped young
d. they both have members that retain the primitive reptile-gait

c. they both gave birth to underdeveloped young

Explanation:

a. they both have nipples for sucking: this is a characteristic that monotremes don't have. They produce milk but babies are fed through skin openings.

b. they both have a single opening for reproduction, excretion and defecation: marsupials have a separate genital tract.

d. they both have members that retain the primitive reptile-gait: Monotremes, like reptiles, have a single cloaca. Marsupials don´t.

Both groups will finished their development inside a mothers sack.