Answer:
b. decrease the number of tasks each employee performs
Explanation:
Obviously, the employees' work load is something that highly affects their productivity. Although it could be possible that the reason doesn't lie in the amount of work, decreasing the existing work load (number of tasks) will surely improve productivity.
Other answers are irrelevant, as the amount of control managers have usually does not affect typical jobs (except highly creative ones). Also, increasing a manager's span of control makes it harder for the manager to help each employee in several tasks.