What keyword is used to define a constant variable in C++?

In C++, the keyword "const" is used to define a constant variable. When a variable is declared with the "const" qualifier, it means that once it is initialized, its value cannot be changed throughout the program. This is particularly useful for creating constants that should remain consistent and unaltered, enhancing code reliability and readability.

For instance, if you declare a constant like this:

const int MAX_VALUE = 100;

The value of MAX_VALUE cannot be modified later in the program, which helps prevent accidental changes and allows developers to work with fixed values clearly defined in the code.

The other options do not serve this purpose. "var" is not a keyword in C++, "static" is used to define variables that maintain their state across function calls or are limited to the scope of a file, and "final" is used in other programming languages (such as Java) to indicate that a variable or method cannot be overridden or changed. Thus, "const" is the appropriate choice for declaring constant variables in C++.