The correct option is C to and fro
In the given statement the correct adverb has to be chosen by looking at the verb which is 'paced', which can be defined as walking at a steady speed, especially when someone is annoyed or anxious. As an adverb describes the verb, the correct answer would be option C which is to walk 'to and fro, i.e. from one point to another, back and forth or up and down. 'Over and over' means to walk frequently; 'far and near' means everywhere; while 'now and again' means occasionally or at intervals. Thus in the given context 'to and fro' is the suitable option because it expresses the restless walking of the person involved within a given area while pondering over a solution. Therefore option A, B and D are incorrect.