Final answer:
Gilderoy Lockhart was treated in the Janus Thickey Ward at St. Mungo's Hospital after losing his memory in "Harry Potter and the Chamber of Secrets."
Step-by-step explanation:
By the end of the school year in "Harry Potter and the Chamber of Secrets," the character Gilderoy Lockhart lost his memory due to an accident with a spell. He was admitted to St. Mungo's Hospital for Magical Maladies and Injuries. The particular ward where Lockhart was treated is the Janus Thickey Ward, which is designated for those suffering from spell damage, including memory loss. Thus, the correct answer to the question is A) Janus Thickey Ward.