Which type of core material decreases inductance when inserted into a coil?

Explanation

A non-magnetic conductive slug moved into a coil carries induced eddy currents whose magnetic field opposes the coil's own, so the net inductance falls; brass is the material customarily used for such tuning slugs and is the one keyed here, and aluminum is the other non-magnetic conductor on the list. Ferrite works the opposite way and raises inductance, while ceramic serves as an inert coil form rather than as a tuning slug.