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If the investor owns no long position in ABC, this represents a “naked call.” As with someone who sells stock short, this investor profits if the market price of ABC drops and loses if the market price rises. Because the maximum market price is unlimited, so is the potential loss. The investor in the question you cite collects $300 to take on the obligation to sell 100 shares of ABC common stock for $55 a share. How much will the investor pay for those 100 shares when/if the contract is exercised?
That is unknown/unlimited, as it is for a short seller who may have to buy the stock in a hurry at whatever the market price happens to be. Remember that writing naked calls and selling stock short both lead to limited gains and unlimited losses. Buying calls and buying stock, on the other hand, are associated with limited losses and unlimited gains.
If the investor owned the underlying stock as you suggest the investor’s maximum loss would be calculated as the difference between what the investor paid to purchase the shares of ABC minus the premium received for selling the 55 call. IF the investor purchased ABC at $52 and sold the 55 call for a premium of $3 the investor’s maximum loss would be $49, calculated as follows: $52-$3 =$49. The investor only received partial protection by employing a covered call strategy. ABC could fall to zero and the investor would suffer their maximum loss of $49 per share. Our videos have entire sections dedicated to showing students how to master option hedging. We provide step by step instructions detailing exactly how to set up these questions so they are very easy to understand.