If the decedent received any of their RMD for the year before death, when preparing the decedent's individual tax return enter the Form 1099-R that was sent to the decedent and indicate as the required RMD only the amount actually take before death.
If trust completed the decedent's remining RMD for the year, simply enter the taxable amount as Other income, line 8 of Form 1041. No need to explain that the distribution satisfied the RMD.