Correct Answer :
D. gain, on
A clever way to tackle this question would be to see the second as a prepositional usage. You go on foot. So, our choices narrow down to (c) and (d). Now, would you be utilising time if you go on foot? Perhaps not. So, (c) is eliminated. If you do something in order to gain time you do it in order to give yourself enough time. For what? To think of an excuse or a way out of a difficult situation.