In which would you expect the best transmission of light? a. hot gas b. a glass of very cold liquid c. colored cloth

You would get the best of transmission of light through hot gases.

Because when we heat up any substance,its density decreases.

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Ohm's law tells you that i = v/r where v is equivalent voltage and r is equivalent resistance of the circuit
since all circuits have the same voltage, your question is really asking which circuit has the lowest total resistance, since the lower the resistance, the higher the current
circuit a has a resistance of 5 ohms; circuit c is in series so has a resistance of 10 ohms; circuit b is in parallel and therefore has a resistance of 2.5 ohms (review your notes on adding resistances in parallel and series); therefore b has the highest current