In electricity supply company uses . . . . . . . . as cost unit.

The correct answer is A. kilo watt hour.

A kilowatt hour (kWh) is a unit of energy equal to one kilowatt of power expended for one hour. It is a common unit of measurement for electricity consumption.

The other options are incorrect because:

• B.
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  per household is not a unit of energy.
• C.
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  voltage is a measure of electrical potential difference. It is not a measure of energy.
• D. None of these is the correct answer.

I hope this helps! Let me know if you have any other questions.