A call (or call option) is a contract that gives the holder the right to buy a stated number of units of the  ¡°underlying¡± asset at a given price (which is called the exercise price or strike prile, here we denote the price by £Ø) from the counterparty  (called the writer of the option).In the case of a European option,this right can be exercised on a given maturity date T,while for an American option it can be exercised at any time until the terminal date T.ª¥
Depending on the underlying asset on which the option is written, a call can be an option on a stock,a stock market index, a currency, a commodity, a bond, or an interest rate,or even a futures contract or a swap (¡°swaption¡±). Currency option is popular.ª¥
A call option allows you to obtain only the ¡°nice¡± part of a forward purchase. Rather than paying £Ø for the foreign currency  (as in a forward purchase),you pay no more than £Ø, and possibly less than £Ø