The following graph explains which of the following concepts?

The correct answer is option 3: Law of variable Proportions

In the graph, the TP increases as labour input increases. But the rate at which it increases is not constant. An increase in labour from 1 to 2 increases TP by 10 units. An increase in labour from 2 to 3 increases TP by 12. The rate at which TP increases is shown by the MP. Notice that the MP first increases (upto 3 units of labour) and then begins to fall. This tendency of the MP to first increase and then fall is called the law of variable proportions or the law of diminishing marginal product. Law of variable proportions say that the marginal product of a factor input initially rises with its employment level. But after reaching a certain level of employment, it starts falling.