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.
