Which of the following is a part of a spark plug?

The correct answer identifies ribs as a component found in a spark plug. Ribs are typically present on the insulator of the spark plug and serve a crucial role by helping to prevent unwanted electrical tracking between the terminal and the body of the spark plug. This tracking can lead to misfires or loss of ignition, so having effective insulation is vital for proper engine performance.

In the context of other parts mentioned in the options, a coil spring is not a component of a spark plug; it's generally used in various mechanical applications. A manifold, on the other hand, refers to a part that distributes air or fuel to different cylinders, and a connecting rod connects the piston to the crankshaft in an engine and is unrelated to the spark plug. Understanding the specific function and structure of spark plugs, including their insulation and structural components like ribs, is essential for diagnosing ignition system issues in small engines.