Final answer:
The counter-charm to Lumos is Nox, which extinguishes the light produced by the Lumos spell. None of the other options provided, such as Expelliarmus, Accio, or Alohomora, function as a counter-charm to Lumos.
Step-by-step explanation:
The counter-charm to Lumos, which is the spell used in the Harry Potter series to produce light from the tip of the caster's wand, is Nox. When the spell Lumos is cast, saying 'Nox' is the magical way to extinguish the light, effectively turning off the light produced by the Lumos spell. None of the other spells listed, such as Expelliarmus, Accio, or Alohomora, serve as a counter-charm for Lumos; they have different uses, like disarming another wizard, summoning objects, or unlocking doors respectively.