Final answer:
Pharmacists should refuse to fill a prescription for an opioid listed as C2 that exceeds the 3-day limit without a valid reason.
Step-by-step explanation:
According to the DEA (Drug Enforcement Administration) regulations, if the prescription for an opioid listed as C2 exceeds the 3-day limit and there is no indication for acute pain exception or non-acute pain, the pharmacist should refuse to fill the prescription. This is because exceeding the limit without a valid reason goes against the guidelines set by the DEA to prevent misuse and addiction.