Identify the correct word from the given options, which means the following:
A place where animals are kept
A. Zoo
B. Aviary
C. Museum
D. Sanatorium

Hint: In such types of questions we need to replace a sentence with an appropriate word. Zoo is an area where animals are kept.

Complete step by step answer:
These type of questions are known as one word substitution in which a sentence is replaced with a single word. Let us try to discuss the given options one by one and try to find out their meanings.
Option A) zoo
This option is correct as zoos are an area where animals, especially wild animals are kept. So a place where animals are kept is known as a zoo. So, this is the correct answer.
Option B) aviary
This option is incorrect as aviary is a large cave where birds are kept as pets. So, this is not the correct word.
Option C) museum
This option is incorrect as a museum is a building where objects of historical, scientific, or artistic interests are kept. Thus option C is incorrect.
Option D) sanatorium
This option is incorrect as 'Sanitorium' is a special type of hospital, usually in the countryside, where people can have treatment and rest, especially when getting better after a long illness. Thus option D is incorrect.

Thus, the correct option is (A).

Note:
One word substitution is the use of one word in place of a wordy phrase in order to make the sentence structure clearer. The meaning, with the replacement of the phrase remains identical while the sentence becomes shorter. Example- one who does not express himself freely- Introvert.