The adverb 'soundly' modifies the verb 'slept'. It tells us about the manner in which the child slept. Hence, Option C - Adverb of Manner is correct.
Option A is incorrect because the adverb 'soundly' doesn't tell us anything about the time when the child slept.
Option B is incorrect because the adverb doesn't indicate any place of reference.
Option D is incorrect because it doesn't speak anything about the frequency of the action.