What type of variable is defined and usable only within certain parts of a program?

The correct answer is local variable. A local variable is a variable that is defined within a function or a block of code, which restricts its scope to that specific area. This means that it can only be accessed and used within the function or block in which it is declared. This encapsulation of the variable helps to prevent interference with other parts of the program, promoting more manageable and organized code.

Local variables are created when the function is called and are destroyed when the function exits, allowing for memory management and reducing the chance of unintended data manipulation outside their intended context. This characteristic makes them essential for functions that need to perform specific tasks independently, without affecting the broader program state.

In contrast, global variables can be accessed from any part of the program, constants are fixed values that do not change, and static variables retain their value between function calls but are still limited in scope to the function they are declared in. Therefore, understanding local variables is crucial for effective programming and code organization.