Yes, according to both Wikipedia (SS vs CAR) and the CIA World Factbook (SS vs CAR), South Sudan is larger than CAR by more than 21,000 sq km, so that answer should be changed.
You don't need to specify "in square miles". The larger country will also be larger in square kilometres, acres, hectares, microns, swimming pools, or Rhode Islands.
I don’t like these quizzes because you have a %50% chance of getting it right and once it’s over you’re like “dang I should’ve got that one!”