Which of the following is a capital expenditure?

A capital expenditure is a long-term investment that benefits a company by enhancing its earning capacity, efficiency, or lifespan. It is not a routine expense, but an investment in a company's future. Out of the given options, "extension of building" is a capital expenditure because it is a long-term investment that will benefit the company by providing more space for operations or storage. The other options - "repairs of generator," "purchase of stock," and "purchase of stationery" - are all examples of operating expenses or current expenses that are necessary for the day-to-day running of the business.