Complete the pet sitter's highly specific instructions for dealing with a very dramatic cat.

Sir Fluffington will still refuse to eat his gourmet salmon ____ you serve it on his fancy crystal platter; he exclusively dines from solid gold bowls.

The correct answer is even if.

"Even if" is used to emphasize that a particular condition (serving the salmon on crystal) will not change the outcome (he will still refuse to eat). "Unless" would imply that serving it on crystal would make him eat it, which contradicts the fact that he only eats from gold bowls.