The law of variable proportion otherwise known as law of diminishing marginal productivity and it states that "if an increasing quantity of a variable factor (labour) is added to the fixed factor (land) the marginal product (MP) and the average product (AP) will first raise reach maximum and start to decline."
In table 10.1, when unit of labour is added to the fixed factor (land), the AP and MP continued to rise up to level of which fourth labour is employed, where they reach 28 and 10 respectively. Those points are the maximum and started too declined.