I like this exercise. What is the definition of a proposition?
A proposition is a mathematical statement such as "3 is greater than 4," "an infinite set exists," or "11 is prime."
With sufficient information, mathematical logic can often categorize a proposition as true or false, although there are various exceptions
FIY: Benson Mates and also here, spent most of one chapter of a textbook saying that there are no propositions just sentences.
BTW, I really don't like the question in the OP.
I was thinking about saying, "Perhaps doing injustice to the philosophy of mathematics..." OK, I can very well accept defining a proposition purely syntactically as a well-formed sentence with no free (unbound) variables.
Why don't you like the question? I think it is important for students to distinguish between terms and sentences and to identify free variables. I often see people writing things like "Induction step: prove k + 1" and using variables that are not properly introduced.