What must be true about a constant during program execution?

A constant in programming is defined as a value that cannot be altered during the program's execution. When a constant is declared, it is given a specific value, and that value is fixed throughout the life of the program. This immutability is crucial for maintaining integrity and reliability in the code, enabling developers to expect that the constant will always represent the same value whenever it is used.

The notion that it must remain unalterable allows programmers to use constants for values that should not change—like mathematical constants (e.g., Pi) or configuration values that are meant to remain fixed throughout the runtime of the application. This characteristic helps prevent unexpected behavior or errors that might occur if a constant's value were allowed to change.

In contrast, the other options refer to different aspects that do not accurately define the nature of constants. Constants are not inherently restricted to a specific data type such as integers, nor are they subject to change or optimization processes by the compiler. The focus on their unchangeable value is what distinctly marks them as constants in programming.