What choke is advised to use with a smooth bore barrel and rifled slugs?

Its called a slug choke you can buy them a Cabelas, Bass Pro Shop, probably online in a bunch of places, and Walmart might sell them but try elsewhere first.

A rifled slug may safely be fired through any choke except 3- an "extra full" choke, which is non-standard, used in turkey shoots- or a variable choke- the fingers of the choke may be broken by a slug- and last- ANY shotgun that the maker says not to use slugs. Some are "overbored"- barrel inside diameter larger than usual- and slug can get a bit sideways. MODIFIED usually give best accuracy, but will vary from gun to gun.