What is the most common special features of science-fiction movies?
"science fiction" is the name of category; most common special features refers to Max(frequency(special_features))
SELECT T1.special_features FROM film AS T1 INNER JOIN film_category AS T2 ON T1.film_id = T2.film_id INNER JOIN category AS T3 ON T2.category_id = T3.category_id WHERE T3.name = 'sci-fi' ORDER BY T1.special_features DESC LIMIT 1