Which of the following insulins can be sold without a prescription?

Humulin Regular is the correct answer because it is classified as an over-the-counter (OTC) insulin in certain forms and formulations. In the United States, Humulin Regular is available for purchase without a prescription, making it accessible to patients who may require it urgently or who do not have immediate access to a healthcare provider. This accessibility allows individuals with diabetes to manage their blood sugar levels effectively.

In contrast, Lantus, Levemir, and Apidra are all example of long-acting and rapid-acting insulins, respectively, that typically require a prescription for dispensing. These types of insulin have more complex dosing regimens and are usually utilized in specific patient populations, thus necessitating professional medical oversight. The prescription requirement for these insulin types ensures that the patient's therapy is appropriately managed and monitored by a healthcare provider, mitigating the risks of improper use and potential adverse effects.