The best way to remove loose interproximal cement on a newly cemented crown is by

using a scaler.
using a piece of floss with a knot tied in the middle.
using an ultrasonic handpiece.
using the air/water syringe.

using a piece of floss with a knot tied in the middle.

Rationale: a piece of floss with a knot tied in the middle. This is very effective in removing a loose piece of interproximal cement after cementation of a crown. The other choices would be better for a piece of cement that has not been dislodged.