Complete the real estate agent's enthusiastic email to a plant-loving client.

You will absolutely love this property! It features a massive _____ in the backyard where you can grow tropical plants all winter.

The correct answer is greenhouse.

A greenhouse (closed compound noun) is a glass building used for growing plants. A green house (adjective + noun) simply means a house that is painted the color green. Since the client wants to grow tropical plants, they need a greenhouse!