The correct answer is: B. first rises and then falls.
The marginal product of labor (MP) is the additional output produced by an additional unit of labor. In the diagram, the MP curve starts out positive and increasing, but eventually reaches a maximum and then begins to decline. This is because as more and more labor is added to a production process, each additional worker has less and less to do, so their output eventually declines.
Option A is incorrect because the MP of labor does not always rise. In fact, it eventually declines.
Option C is incorrect because the MP of labor does not always decline. In fact, it first rises and then declines.
Option D is incorrect because the MP of labor does not first fall and then rise. It first rises and then declines.