A call option is a right to buy without an obligation to buy. So if you have a call option on TCS then you have the right to buy TCS but no obligation to buy TCS at a pre-determined price. For example, if you have bought a TCS 1-month 2700 call option at a price of Rs.45. On the settlement day if the price of TCS is Rs.2850, the option is profitable to you. But if on that date the price of TCS is Rs.2500 then you are not interested in buying TCS at 2700 when you can buy it in the open market at Rs.2500. For this right without obligation you pay a premium of Rs.45, which will be your sunk cost.

A call option is a right to buy without an obligation to buy. So if you have a call option on TCS then you have the right to buy TCS but no obligation to buy TCS at a pre-determined price. For example, if you have bought a TCS 1-month 2700 call option at a price of Rs.45. On the settlement day if the price of TCS is Rs.2850, the option is profitable to you. But if on that date the price of TCS is Rs.2500 then you are not interested in buying TCS at 2700 when you can buy it in the open market at Rs.2500. For this right without obligation you pay a premium of Rs.45, which will be your sunk cost.