In which one of the following circumstances could mechanical energy not possibly be conserved, even if friction

Question:

In which one of the following circumstances could mechanical energy not possibly be conserved, even if friction and air resistance are absent?
(a) A car moves up a hill, its velocity continually decreasing along the way.
(b) A car moves down a hill, its velocity continually increasing along the way.
(c) A car moves along level ground at a constant velocity.
(d) A car moves up a hill at a constant velocity.