Select the option that can be used as a one-word substitute for the given group of words.

A place where airplanes are kept for maintenance

The correct answer is Hangar.

Let's break down the meaning of the given options to find the correct one-word substitute:
1. Hangar: A large building, typically a shed, used for housing and maintaining aircraft (airplanes, helicopters, etc.). This matches the definition given in the question.
2. Hanger: A shaped piece of wood, plastic, or wire with a hook at the top, used for hanging clothes. Note the spelling difference ("-er" vs. "-ar").
3. Scullery: A small room at the back of a house or kitchen, used for washing dishes and other dirty household work.
4. Aviary: A large cage, flight, or building for keeping birds.

Therefore, the appropriate one-word substitute for "A place where airplanes are kept for maintenance" is Hangar.