IMO C

1) Avg sal of Mgr = Avg sal of ALL - 5000. We don't know about directors. Insufficient.

2) Avg sal of Dtrs = Avg sal of ALL + 15000. We don't know about managers. Insufficient.

Combined,
You need to find the percentage here. If you are asked to find the number of managers or directors, then it would be E.

If the overall average is near an entity (In this case, managers and directors are entities), then the number of that entity is larger.
MGR-----Overall Avg--------------Directors
5000---- OVERALL -------------- 15000

Distance b/w managers and directors is 20000 units. Distance b/w managers and Overall Avg is 5000. Distance b/w Directors and Overall Avg is 15000.

So, the percentage of directors is 5000/20000 = 1/4.

Sufficient.