The correct option is
D gently
We must read the poem in order to be able to fill in the first blank. "Wind, come softly", here the poet addresses the wind and asks it to come softly. Let's fill the blank with the synonym of 'softly'. Here 'softly' means in a calm and not rough manner.
Option A - 'Carefully' is an adverb, which means to do something cautiously. Carefully and softly don't have the same meaning. Hence, option A is incorrect.
Option B - 'Slowly' is an adverb, which means at a low speed. As slowly and softly don't have the same meaning, option B is incorrect.
Option D - 'Violently' is an adverb, which means strongly and forcefully, in a destructive manner. Violently and slowly don't have the meaning. Hence, option D is incorrect.
Option C - 'Gently' is an adverb, which means in a soft manner. It's the synonym of 'softly', hence option C is correct.