Explanation:

A request is a sentence that asks someone to do something or to provide something. It is a polite way of asking for assistance or a favor. In this question, we need to identify the sentence that is a request among the given options.

The correct answer is option 'A' - Please pass me the salt.

Explanation of the answer:
To explain the answer in detail, let's break it down into headings and bullet points.

1. Understanding the concept of a request:
- A request is a sentence that asks someone to do something or to provide something.
- It is a polite way of asking for assistance or a favor.

2. Analyzing the options:
Let's analyze each option to determine if it is a request or not.

a) Please pass me the salt:
- This sentence is a polite way of asking someone to hand over the salt.
- It clearly expresses the speaker's desire for the salt to be given to them.
- It meets the criteria of a request as it asks someone to do something.
- Therefore, this sentence is a request.

b) Don't touch that!:
- This sentence is an imperative statement that tells someone not to touch something.
- It is a command or a prohibition, not a request.
- Therefore, this sentence is not a request.

c) You should finish your homework:
- This sentence is a statement of advice or recommendation.
- It suggests that someone should complete their homework.
- It does not directly ask someone to do something.
- Therefore, this sentence is not a request.

d) Bring me a glass of water:
- This sentence is a direct instruction or command.
- It tells someone to bring a glass of water to the speaker.
- It does not use polite language to ask for assistance.
- Therefore, this sentence is not a request.

Conclusion:
- After analyzing each option, it is clear that the sentence "Please pass me the salt" is the only one that meets the criteria of a request.
- It is a polite way of asking someone to hand over the salt.
- Therefore, the correct answer is option 'A'.