The correct option is C to visiting
Let’s examine the sentence: “I look forward ____ you soon.” The blank requires ‘to’ as ‘look forward to’ is a widely used phrasal verb. This rules out Options A and D as they do not have ‘to.’ Further the sentence requires a noun or a gerund referring to the event of ‘visiting’ rather than the action of ‘visiting.’ The gerund always has the same function as a noun (although it looks like a verb). The gerund ends in ‘ing.’ As Option B fulfils this requirement, so it is the correct answer.