Final answer:
The sentence "Drug companies have begun to prohibit the use of their products in executions" expresses a verifiable fact, as it states an action taken by drug companies regarding their products' use.
Step-by-step explanation:
The sentence that expresses a fact rather than an opinion from the options given is: "Drug companies have begun to prohibit the use of their products in executions." This statement can be verified by looking into the policies of drug companies regarding their stance on the use of their products in executions. It presents an observable action, not a subjective viewpoint. The other sentences could involve some level of personal belief, interpretation, or are subjective, making them opinions rather than concrete facts.