Explanation:
Air fuel ratio:
Air fuel ratio is the ratio of mass of air to the mass of fuel.So we can say that

As we know that fuel burn in the presence of air that is why we have to maintain a proper amount of air fuel ratio.
When we need more power then we have supply more fuel and to burn this fuel ,require a specified amount of air.So for different loading condition of engine different air fuel ratio is required.
When air is less and fuel is more then it is called rich air fuel ratio .when air is more and fuel is less then it is called poor air fuel ratio.
Answer:
I couldn't find options for your question online, but I can give you an explanation so you can choose the correct option.
Explanation:
A spark knock is a form of unpredictable behavior that occurs in combustion, that is, in the chemical reaction that occurs between oxygen and an oxidizable material. Such combustion is usually manifested by incandescence or flame.
The spark knock is a detonation that occurs when there is a lot of pressure in the fuel.
<u>Some situations in which this can happen are:
</u>
- Engine overloaded.
- Maximum pressure in the cylinders.
- Engine overheated.
- Overheated air.
- Long and excessive engine ignition timing.
- Spark plug at high temperatures.