The correct answer is: D. adjust his sales in the two markets, so that his MR in each market just equals his marginal cost.
A profit-maximizing monopolist will always produce at the point where marginal revenue equals marginal cost. This is true regardless of whether the monopolist is selling in one market or two markets. In the case of two markets, the monopolist will adjust his sales in each market so that the marginal revenue in each market is equal to the marginal cost. This will ensure that the monopolist is maximizing his profits in each market.
Option A is incorrect because the monopolist will not always charge a higher price in the less selling market. If the demand in the less selling market is more elastic, then the monopolist will actually charge a lower price in that market.
Option B is incorrect because the monopolist will not always charge a higher price in the more selling market. If the demand in the more selling market is more elastic, then the monopolist will actually charge a lower price in that market.
Option C is incorrect because the monopolist will not always charge the same price in both markets. The monopolist will charge the price that maximizes his profits in each market, which may not be the same price in both markets.