Electric current in solids flows via electrons

Steps:

• Identify conductors: copper is a metal with free electrons; graphite has delocalized electrons.
• Recall mechanism: in solid conductors, current results from electron movement, not ions.
• Confirm for both: electrons drift through lattice in copper and between carbon layers in graphite.
• Conclude: only electrons move to complete circuit and light bulb.

Why B is correct:

• By definition, electric current in metallic and graphitic solids is the directed flow of free electrons (Ohm's law: I = V/R, where charge carriers are electrons).

Why the others are wrong:

• A: Ions like copper or carbon do not move in solid conductors; only electrons do.
• C: Ions are immobile in solids, unlike in liquids or electrolytes.
• D: Carbon ions do not exist or move; graphite conducts via electrons alone.

Final answer: B