When does a constant differ from a variable?

A constant differs from a variable primarily in that a constant is established with a specific value that remains unchanged throughout the duration of a program's execution. This means once a constant is set, its value cannot be altered or reassigned, ensuring that it retains the same data point for the entire lifecycle of that program. This property makes constants particularly useful for defining values that should remain consistent, such as mathematical constants (like π) or fixed configuration parameters.

In contrast, variables are designed to allow for modification of their values during the program's execution. They are flexible and can be reassigned or updated as needed based on program logic or user input, which is essential for dynamic applications.

Options that suggest a constant can change its value or is defined at runtime misrepresent the fundamental characteristics of constants in programming. Moreover, the assertion that constants cannot hold complex data types is misleading, as constants can indeed store complex data types, provided they are initialized correctly and permitted by the programming language in use.