The missing word is “charged”.

Quick Scoop: Why ions conduct electricity in electrolysis

When exam questions say:

“Ions can conduct electricity during electrolysis because they are free to move and they are… what?”

they are usually looking for:

  • Free to move
  • Charged particles

So the full idea is:

  • Ions are charged , so they can carry electrical charge through the liquid or molten electrolyte.
  • Because they are free to move , these charges can flow, which is what we call an electric current.

In many GCSE/IGCSE mark schemes, this is phrased as:

  • “Ions act as charge carriers in the electrolyte.”
  • “Ions are free-moving charged particles.”

So the best one‑word completion is:

“…free to move and they are charged.”

TL;DR: Ions conduct electricity during electrolysis because they are free‑moving charged particles.