Which sentence is written correctly? A. Steven solved the puzzle quicker than I did. B. The children looked more calmer after their nap. C. His pitching is the most accuratest on the team. D. A hammer is one of the most handiest tools.

Only sentence A is correct. As 'quick" is a monosyllable the comparative is formed by adding 'er'

Calm is a monosyllable too. So you don't need to put 'more'before 'calmer'

Ac-cu-rate has 3 syllables, so 'est' can't be added to the adjective. It should be - most accurate.

Adjectives that end in 'y' form the comp. and superlative like mono syllables without 'more' or 'most'-It is the handiest tool