Which statement properly defines a constant with the value 123?

The correct choice defines a constant using the keyword 'final,' which is appropriate in languages like Java. When you declare a constant with 'final', it indicates that the variable's value cannot be modified after it's been assigned. In this case, declaring 'final int MY_CONST = 123;' succinctly initializes the constant MY_CONST with the value of 123, ensuring that it remains unchanged throughout its scope.

Using 'final' is crucial for constants, as it enforces the immutability rule in the programming language, preventing unintentional changes elsewhere in the code. This makes the code easier to read, maintain, and debug because constants are often used for values that need to remain consistent, like mathematical constants or configuration values.

The other options are incorrect due to the misuse of keywords or syntax. Some options may imply the intention of declaring a constant, but they either allow for modification or use keywords not suited for constant declaration in the given context.