When can additional costs be applied to a treatment option?

The accurate answer indicates that additional costs can be applied to a treatment option if a liquid treatment becomes necessary. This scenario often arises when the initial method proposed is not sufficient to effectively address the specific pest issue at hand.

In pest management, different treatment modalities have varying levels of effectiveness based on the type and severity of the infestation. If it becomes clear that a more intensive or specialized method, such as a liquid treatment, is required to ensure complete eradication of pests, then it is justifiable to apply additional costs. This ensures that the client receives a treatment that is adequate to handle the conditions rather than settling for a less effective solution that could lead to recurrence of the problem.

In contrast, the other choices do not accurately reflect the policy regarding when additional costs might be warranted. For instance, a customer's request does not necessarily dictate whether costs should increase, nor does the duration of the treatment directly influence pricing. Similarly, while a severe infestation can justify more robust treatment measures, the key factor that aligns with the specific scenario in the question is the necessity of switching to a liquid treatment due to its requirement for effective pest control.