Explanation

A is correct because 100 watts used for 10 hours equals 1 kWh. B is incorrect because 100 watts used for 5 hours equals 0.5 kWh. C is incorrect because 100 watts used for 8 hours equals 0.8 kWh. D is incorrect because 100 watts used for 10 hours equals 1 kWh, but the question asks when it consumes one unit (kWh), and A represents this calculation correctly.