In the following question, there is a sentence which is divided into four parts. Choose the fragment that carries an error (if any). Ignore punctuation errors if any.
A growing body of evidence is supporting the "use-it-or-lose-it" hypothesis of aging well. / It says staying mentally active helps keep the brain in shape. / So far, research has focused on reading books, / crossword puzzles and learning languages.
crossword puzzles and learning languages.
Parallelism error-reading, learning languages are in ing form. but crossword puzzles (a noun) is not parallel. It should be changed to doing crossword puzzles. We can also make the sentence parallel by changing it to: focused on books, crossword puzzles and languages. Then the error will be in two fragments. Hence, it is not possible to work the question that way.