The correct answer is: A. less than the marginal productivity of labour.
A monopoly in the product market is a market structure in which there is only one seller of a good or service. A monopsony in the labor market is a market structure in which there is only one buyer of labor.
In a monopoly market, the monopolist has market power and can set the price of its product. This means that the monopolist can pay workers less than their marginal productivity of labor.
In a monopsony market, the monopsonist has market power and can set the wage of labor. This means that the monopsonist can pay workers less than their marginal productivity of labor.
Therefore, when there is monopoly in the product market and monopsony in the labor market, wages will be less than the marginal productivity of labor.
Option B is incorrect because the monopolist and monopsonist will not pay workers more than their marginal productivity of labor. If they did, they would lose money.
Option C is incorrect because the monopolist and monopsonist will not pay workers equal to their marginal productivity of labor. If they did, they would lose money.
Option D is incorrect because the wages set by the trade unions are not necessarily equal to the marginal productivity of labor.