How much can I lose buying a call option?
If you buy a call and hold it, the most you can lose is what you paid for it (the premium), plus fees. If the stock ends at or below the strike, the call expires worth nothing; however far the stock falls, the loss stops there. Exercising the call, selling options instead of buying them, or borrowing to trade are different, and can lose more.
Try it: a call on a $50 stock, strike $50, bought with 30 days left at 30% implied volatility. Move the stock's price at expiry.
Simulated example, priced by the StrikePath engine (Black–Scholes, interest 4.3% a year). For learning; not advice.
Why the loss stops at the premium
A call is the right, not the duty, to buy shares at the strike. If the stock ends below the strike, you simply don't use the right: it expires worthless, and what you paid is gone. You never have to buy shares at a loss.
One thing to know: if you hold a call to expiry and it ends above the strike, your broker will usually exercise it for you, and you will own 100 shares, with the risks below. Selling it before expiry avoids that.
What can make it more
If you exercise a call, you buy 100 shares at the strike and now own shares, which can fall further. If you sell (write) a call without owning the shares, the loss has no ceiling, because a stock's rise has none. And a premium paid with borrowed money is still owed. The lesson on selling shows the other side.
A whole loss is common
Losing the whole premium is not a rare outcome for a bought call: whenever the stock ends at or below the strike, that is the result. How often that happens depends on the strike, the time left and how much the stock moves; the course's practice flights let you see it many times on simulated markets.
What this shows, and what it doesn't
It shows the result at expiry with made-up numbers. Before expiry, the call's value also depends on time and implied volatility, so before expiry it is worth more than the value at expiry shown here. Fees and the bid-ask spread add to any loss.