The correct answer is: (R) is appropriate reason of (A).
The assertion (A) states that as the proportion of one variable factor in a combination with fixed factor is increased, after a point the marginal product of the factor will diminish. This is because as more of the variable factor is added, it will have less and less of an effect on the output, as the fixed factor is already at its maximum capacity.
The reason (R) states that beyond the level of optimum combination of inputs leads to this. This is also true, as the optimum combination of inputs is the point at which the marginal product of each input is equal. After this point, adding more of any input will cause the marginal product of that input to decrease.
Therefore, (R) is an appropriate reason of (A).