Step 1: Work out the relationship between the first pair.
An "incubator" is a controlled, protective environment used to nurture a fragile, newly born "infant" until it is strong enough to survive on its own.
Step 2: Apply the same relationship to the second pair.
A "green house" is likewise a controlled, protective environment, but built to nurture young, delicate plants until they are hardy enough to be moved outdoors. We need the option that names the young, tender form of a plant.
Step 3: Eliminate the options that do not fit.
(a) Parasites are organisms that live off a host, they are not raised or protected in a greenhouse.
(b) Worms and (d) Tadpoles are young animal forms, not plant forms, so they do not belong in a greenhouse.
(c) Puppies are young dogs, an animal again, unrelated to a greenhouse's purpose.
Step 4: Confirm the best match.
(e) Saplings are young, tender plants or trees, exactly the kind of fragile growth a greenhouse is designed to nurture and protect, just as an incubator nurtures an infant.
Step 5: State the answer.
The correct answer is option (e), Saplings.