%_MC question
The Greek letter $\xi$ is written in English as

%_MC answer_A
xi

%_MC answer_B
eta

%_MC answer_C
phi

%_MC answer_D
zeta

%_MC answer_E
chi

%_MC explanation
Although only used when most of the other letters are already taken, $\xi$ is a full-fledged member of the Greek alphabet and deserves respect.