Technically, any monthly pension payment is considered a Required Minimum Distribution (RMD) and the entire amount is an RMD. That is how you need to answer the questions.
All retirement accounts, even pensions, must meet the RMD requirements. However, the way the pension plans meet the RMD requirements is by creating an annuity paid periodically (in your case monthly.
When asked "Was this withdrawal an RMD?", the answer is "Yes", then check "All of this distribution is an RMD."
(You would get the same result if you answered "No" when asked "Was this withdrawal an RMD?", and the interview will skip the next question about how much of the withdrawal was for the RMD).
