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Stock Option is an option whose underlying asset is a stock. In the case of call stock option, its holder has the right, but not the obligation, to buy the underlying stock from the writer before its expiration date. In the case of put option, its holder has the right, but not the obligation, to sell the underlying stock to the option writer.
There are options known as American and European. American options can be exercised anytime before expiration date, while European options can be exercised only on the date of expiration. However, most stock options that are traded on the exchange are American options.