Which sentence is written correctly? a. jackie spoke more longer than any other speaker. b. isaac's version of the story is more nearer to the truth than hers is. c. last winter was more snowy than this winter. d. vanessa's room was the most cleanest of any teenager's room.

When writing a sentence with comparatives and superlatives, only one should be included; it usually comprises of the word's root and -er / -est.

Because of that, you can deduce that A is wrong ("more" should be deleted; "longer" already conveys the message. B is also incorrect (same story with "more nearer"). And D is incorrect ("most cleanest" is wrong). This leaves you with C, which makes sense since "snowy" does not come in comparative mode; there is no "snowier", which requires the need of "more" to contrast the two winters. 

The answer is accept it