Question

Which sentence uses the correct comparative form of the adjective pretty? A. That blue dress is the prettiest one on the rack. B. This blouse is prettier than the one that got stained. C. Those flowers are very pretty. D. I think yellow roses are more prettier than red roses.

Answer 1

Usually a comparative adjective takes on the ending -er
Therefore you can eliminate A and C from the list.
You are left with B and D, however in D it says more prettier which is incorrect grammar therefore it must be... 
B