Since you will no longer be married in 2024 then select Single on the W-4P. You only would be a qualified surviving spouse if you have a qualified child as your dependent.
For your 2023 tax return you can file as Married Filing Jointly since your spouse passed away in 2023.
For Step 2 just use your pension and Social Security income. Unless your wife also had a pension and you are a beneficiary of the pension then add that pension income to yours.
