Why are lead compounds added in petrol?

Answer: Option A

Knocking is an undesirable phenomenon in petrol engines.
It happens when the air-fuel mixture in the cylinder ignites prematurely and uncontrollably.
This creates pressure waves that can damage the engine and reduce its efficiency.

Lead compounds, like tetraethyl lead (TEL), were historically added to petrol to increase the octane rating.
A higher octane rating makes petrol more resistant to knocking.

Therefore, the primary reason for adding lead compounds was to reduce or prevent knocking.

The other options are incorrect because:
Option B: While some additives *might* have a minor effect on HC emissions, it wasn't the main reason for using lead.
Option C: Lead compounds don't directly reduce exhaust temperature.
Option D: Lead compounds primarily improved engine efficiency by preventing knocking, but didn't directly "increase power output" as their main function.