which of these sodium, silicon, tetrachloromethane, argon or potassium bromide?

Question

could be described as a substance which is decomposed by an electrical current in the liquid state

Answer 1

The Potassium Bromide.
It is a salt, it is made out of positive and negative ions, and when you put current through it, the ions will separate. It has to be liquid to conduct electricity at all.
All other substances you mention consist of molecules held together by covalent bonds, and they don't split under current. Argon is the exception, it doesn't consist of molecules but of single atoms, you can't decompose it except you get some kind of nuclear reaction (very improbable that there is one for argon which is possible on Earth).

Answer 2

Umm..like in a hot water heater?