Explanation:

In order to understand why option 'D' is the correct answer, let's first review the concept of profit maximization for a monopolist.

A monopolist is a single seller in the market who has control over the supply of a particular product or service. Unlike in a competitive market, where firms are price takers, a monopolist has the ability to set the price of the product.

To maximize profits, a monopolist needs to find the level of output where the difference between marginal revenue (MR) and marginal cost (MC) is the greatest. This is because MR represents the additional revenue earned from selling one more unit of the product, while MC represents the additional cost of producing one more unit.

Key Points:
1. Profit maximization occurs when MR = MC.
2. MR is the change in total revenue resulting from selling one more unit.
3. MC is the change in total cost resulting from producing one more unit.

Explanation of Option 'D':
The correct answer, option 'D', states that a monopolist is able to maximize his profits when his marginal cost is equal to marginal revenue.

When MR is greater than MC, it means that the additional revenue earned from selling one more unit is greater than the additional cost of producing that unit. In this case, the monopolist should increase production to maximize profits.

On the other hand, when MC is greater than MR, it means that the additional cost of producing one more unit is greater than the additional revenue earned from selling that unit. In this case, the monopolist should decrease production to maximize profits.

Therefore, the point where MR equals MC is the optimal level of output for profit maximization. At this point, the monopolist is producing the quantity of goods where the cost of producing one more unit is exactly equal to the revenue earned from selling that unit. Any deviation from this point would result in lower profits.

Conclusion:
To summarize, a monopolist is able to maximize his profits when his marginal cost is equal to marginal revenue. This is the point where the monopolist is producing the optimal level of output, ensuring that the additional cost of production is equal to the additional revenue earned from selling one more unit.