Answer: C. a mental set
Explanation: The development of a rigid perception or inclination about the a methodology or approach simply because such approach has proved efficient at a certain point in time. The persistence in the use and application of this past idea or approach even when it is no longer the most efficient could be said to be caused by a mental set. In the context above, the street to the north provided a route for Amber after the street to the south was closed. Having used the route for three months during the closure period of the shorter street to the south, Amber developed a mental set causing him to be inclined or attached to taking the street to the north even after the shorter southern street was reopened.