The negative in a = -2 for answer A represents a reflection over the x-axis.
What does the cosine graph usually look like?.....It goes through point (0,1) and decreases.
The graph shown goes through point (0,-2). Since there are no vertical or horizontal shifts, the graph has an amplitude of 2 and reflects over the x-axis.
a=2 for answer D does not represent the reflection over the x-axis.