To identify which engine problem is indicated by high carbon monoxide levels in the exhaust, let's analyze the options provided:

1. Air-fuel mixture is too rich in the engine
2. Poor tire alignment during operation
3. Worn spark plugs in the ignition system
4. Low coolant level in the radiator

Analysis and Explanation:

• Air-fuel mixture is too rich in the engine: A high level of carbon monoxide (CO) in the exhaust is typically a sign that the air-fuel mixture is too rich. This means there is more fuel than necessary in the combustion chamber, which leads to incomplete combustion and hence, increased carbon monoxide emissions. This is because carbon monoxide is a by-product of incomplete combustion when there is not enough oxygen to convert all the carbon in the fuel to carbon dioxide.
• Poor tire alignment during operation: Tire alignment issues affect the handling and tire wear of a vehicle but do not influence the composition of exhaust emissions, hence not related to CO levels.
• Worn spark plugs in the ignition system: While worn spark plugs can lead to incomplete combustion and misfires, which might increase emissions, the specific symptom of high CO levels is more directly associated with a rich fuel mixture than with solely spark plug wear.
• Low coolant level in the radiator: Low coolant would primarily cause overheating and engine performance issues but does not have any direct correlation to exhaust CO levels.

After considering all options, the correct answer is that high carbon monoxide levels in the exhaust indicate that the air-fuel mixture is too rich in the engine. This option directly relates to the exhaust emissions composition.