The correct answer is option 4, Darjeeling. This means that Darjeeling was the first city in India to use electricity commercially. The other options provided, Kolkata, Chennai, and Mumbai, did not use electricity commercially before Darjeeling. It is worth noting that although Darjeeling was the first city in India to use electricity commercially, it may not have been the first place in India to have access to electricity. This distinction is important because electricity may have been available in other parts of India for non-commercial use, such as in factories or homes, before it was used commercially in Darjeeling.