How do I calculate how well a liquid conducts electricity?

[j.s.thomas]

Which one of the following substances would
likely be the best conductor of electricity in
the pure liquid state (that is, when melted)?
1. bromine (Br2)
2. barium fluoride (BaF2)
3. water (H2O)
4. phosphorus trichloride (PCl3)
5. methane (CH4)



I don't know any relevant equations, I really would prefer to be taught how to solve this, than just get the answer

 

[Borek]

Do you know how the current can be conducted? What must be present?

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methods