What is the preferred scope for a pump rifle?
Same as anything you would use on a comparable bolt action.
Right now I have a straight tube 1.5x Burris FF on my carbine. There is a Swift 1.5-4x that will probably end up on it.
I have used 3-9x in both 32 and 40mm on them. Ex FIL had a 1.75-5x Redfield on his, dandy little scope.