You need the market data subscription just to see the price of the option and the underlying stock NOT to exercise an option. The exercise option functionality comes with TWS. And mind you, if you do not want to exercise the option, you can always sell the option or instruct IB NOT to exercise the option, that you can do in the "option exercise" functionality in TWS as well provided you do it by latest before 4:15 PM EST/EDT on the expiration date. You are never forced to exercise the option ever, just remember that. An option is just what it sounds like. An option pun intended.