Fluoride recharging of a glass ionomer cement restoration is more effective by which of the following agents?

The ability of a restorative material to be recharged and serve as a fluoride reservoir depends on the type and permeability of the material, the frequency of fluoride exposure, and the concentration of the fluoridating agent. For example, recharging is more effective by acidic fluoridating agents such as APF. Ref: Phillip’s Science of Dental Materials ed 12 pg 334