I have seen proofs of whether a set of connectives is functionally complete, but never when it is defined by a truth table. I can't quite figure out how to show that {△} is functionally complete if the propositional connective is defined by a truth table.
Here is an image of the truth table.
Reduce the problem to one you already know how to solve: identify which operation $\Delta$ is in your favorite method of describing them, and then use the method you already know (or reference facts you already know) to see that its functionally complete.