Which statement is true about variables?

Variables are fundamentally defined as symbols or placeholders in mathematics and programming that represent values that can change. This is why the statement about their ability to change depending on the context in which they are used is accurate.

In mathematics, for example, a variable such as "x" can stand for different numbers in different equations or scenarios. Similarly, in programming, a variable can hold different values at different times during the execution of a program. This flexibility is what makes variables so useful, as they allow for generalization and the ability to represent a range of values rather than a single, fixed number.

The other statements do not accurately capture the nature of variables. Limiting their expression solely to letters overlooks the fact that variables can also include other symbols in various contexts. The idea that they always represent the same value contradicts the very definition of what a variable is meant to do. Lastly, claiming that variables are not used in mathematical expressions disregards their crucial role in expressing relationships and operations within equations.