The "correct" answer is so wrong in so many ways. It asks me if I'm smarter than the ambigious "one week ago." I realize how it would be interpreted in casual conversation, but it doesn't make the grade in written communication.
The “last week” (poslední týden) is a period of time that extends over seven days (in Czechia from Monday to Sunday). “Před týdnem” is a date or moment within that period. So it's “a week ago” in English.