Introduction:
Expressions are an important concept in mathematics. They consist of variables, constants, and operations. In this question, we are asked to identify the components of an expression.

Explanation:
An expression is a mathematical statement that combines variables, constants, and operations. Let's break down the components mentioned in the options:

Variables:
- Variables are symbols that represent unknown values or quantities in an expression.
- They can be represented by letters, such as x, y, or z.
- Variables can take on different values, and their values can vary depending on the context.
- For example, in the expression 2x + 3, x is a variable.

Constants:
- Constants are fixed values that do not change in an expression.
- They are represented by specific numbers, such as 2, 3, or 5.
- Constants have a fixed value and do not depend on any other variables.
- For example, in the expression 2x + 3, 2 and 3 are constants.

Identifying the correct answer:
Based on the given options, the correct answer is option 'A' - variables and constants. This is because expressions consist of both variables and constants. Variables represent unknown values or quantities, while constants represent fixed values. Therefore, option 'A' is the most appropriate choice.

Summary:
In summary, expressions consist of variables and constants. Variables represent unknown values or quantities, while constants represent fixed values. These components are essential for creating and understanding mathematical expressions.