Why does the marginal revenue product(MRP) curve slope downward for a perfectly competitive firm?

Question

a. because MRP=MR x MPP. After some point, as more of a factor is employes, the lower its MFC is; thus MRP declines

b. Because MRP = MFC x MPP. After some point, as more of a factoc is employed, the lower its MFC is; thus MRP declines.

c. Because MRP = MR x MPP. After some point, MR declines for a product price taker; thus, MRP declines.

d. Because MRP = MFC x MR. After some point, MFC and MR decline; thus, MRP declines.