Consider a firms per-period (e.g., hourly) production process. If it employs 1 unit of labor, then 8 units of output will be produced; if it employs 2 units of labor, then 12 units of output will be produced; and if it employs 3 units of labor, then 15 units of output will be produced. It follows that:
total output is increasing at an increasing rate and the marginal product of labor is decreasing
total output is increasing at an increasing rate and the marginal product of labor is increasing
total output is increasing at an increasing rate and the marginal product of labor is constant
total output is increasing at a decreasing rate and the marginal product of labor is increasing
total output is increasing at a decreasing rate and the marginal product of labor is decreasing
total output is increasing at a decreasing rate and the marginal product of labor is constant