It is so simple. where is the confusion ?
the graph of y = 2.5 x is clearly the graph B.
slope is 2.5 > 1, it means it is positive, and steeper than y = x.
the graph is raising as you move to the right, as slope is positive.
as slope is > 1, y increases faster than x.. it is more vertical than horizontal.
at x = 2, y = 2.5 * 2 = 5 units, it is clearly seen in the graph.