The correct answer is C) Worn piston rings. When one cylinder has lower firing and compression pressures compared to the others, the most likely cause is worn piston rings. Worn piston rings allow compression and combustion gases to leak past the piston, resulting in lower pressures in that cylinder. This is because the piston rings play a critical role in sealing the combustion chamber and maintaining proper compression and combustion pressures. The other options are incorrect because early or late fuel injection would affect all cylinders equally, and low engine load would not cause uneven cylinder performance.