Refer to figure.
In light aircraft, the cabin heating is supplied by a basic system whereby outside air enters through a simple inlet, passes through a chamber (shroud) that surrounds the hot engine exhaust, is heated up and channelled in to the cabin. This system works really well as a simple, cheap and effective system. However!

Carbon monoxide is a poisonous gas that is produced through incomplete combustion. Aircraft engines (as well as cars, lorries, and any other internal combustion engine) will produce it, as they are quite inefficient. As long as the exhaust is not damaged, cabin air and exhaust gas will be kept separate. But if the exhaust is damaged and the gas leaks in to the heating shroud, the two will combine and potentially lethal air will enter the cabin.

Carbon monoxide is also inhaled when you or others are smoking, and is just as poisonous in that form.

This question is sneaky, as there are technically three correct answers - the only truly incorrect option is a leaking fire extinguisher, which may produce poisonous gas (if it is a BCF/Halon/Freon version) but that gas is not carbon monoxide. You ned to pick the most correct answer - both a leak in the cabin air / exhaust system heat exchanger AND cigarette smoke. Individually the options are not wrong, but the more correct answer is the one that contains both.