Which of the following is proposition?

A proposition is a declarative statement that is either true or false. To determine if a sentence is a proposition, check if it is a statement (not a question, command, or expression with variables) and has a definite truth value.

Option A: "What is your name?" is a question, not a statement, so it is not a proposition.

Option B: "x + 2 = 5" contains a variable. Its truth value depends on x, so it is not a proposition (it is a predicate).

Option C: "The sky is blue" is a declarative statement that can be evaluated as true or false, so it is a proposition.

Option D: "Walk to the store" is a command, not a statement, so it is not a proposition.