If the firms are competitive and profit maximizing, the demand curve for labour is determined by which one of the following?

Correct option is B

Marginal product of labour (MPL) refers to the change in output that results from adding a unit of labour. It is a feature of the production function and depends on the amounts of physical capital and labour already in use.
In a competitive and profit-maximizing firm, the demand for labor is determined by the value of the marginal product of labor (VMPL). The VMPL represents the additional output or revenue generated by hiring an additional unit of labor.
When a firm considers hiring an additional worker, it assesses the contribution of that worker to the firm's production process. If the value of the additional output or revenue generated by the worker is higher than the cost of hiring that worker, it is profitable for the firm to hire them.
The firm's demand for labor is derived from this cost-benefit analysis. It will continue to hire additional units of labor as long as the VMPL is greater than or equal to the wage rate. The firm will hire workers up to the point where the VMPL equals the wage rate, as hiring additional workers beyond that point would result in diminishing returns and lower profitability.
Therefore, in a competitive and profit-maximizing firm, the demand curve for labor is determined by the value of the marginal product of labor. The VMPL reflects the productivity and contribution of labor to the firm's output and revenue, and it guides the firm's decision-making regarding the quantity of labor to employ.