Solution:According to the law of variable proportions, when the variable factor is increased in the short-run, then marginal product first increases, then starts to fall and can become negative also. This law is characterised by the presence of three stages, i.e.
Increasing return to factor, diminishing returns to factor and negative returns to factor. In the third stage, total product falls and marginal product becomes negative. Hence, option (d) is not correctly matehed