Final answer:
The millionaire who hired the young Gatsby as an assistant is Dan Cody.
Step-by-step explanation:
The correct answer is (D) Dan Cody. In F. Scott Fitzgerald's novel The Great Gatsby, the young Gatsby is hired as an assistant by the millionaire Dan Cody. Cody serves as a mentor and father figure to Gatsby, and his wealth and extravagant lifestyle greatly influence Gatsby's desires and ambitions.