In order to minimize the net cost of pollution reduction, the level of pollution reduction should be the level at which

The marginal benefit that gained by reducing pollution should be more than marginal cost used to reduce pollution.

Explanation:

In order to reduce the net cost for pollution reduction the marginal benefit should be more than marginal cost. If the marginal benefit is more than marginal cost then it will be good for the society. Marginal benefit reduces when there is increase in consumption, when the marginal cost is more than marginal benefit which means by consuming one extra unit the consumer is not getting the satisfaction.