Before installing spark plugs, which of the following should be done?

Adjusting the electrode gap before installing spark plugs is crucial for optimal engine performance. The electrode gap affects the spark plug's ability to create a strong, consistent spark, which is essential for ignition. If the gap is too wide or too narrow, it can result in misfiring, poor fuel economy, reduced power output, and increased emissions. Therefore, ensuring the proper electrode gap based on the manufacturer's specifications is a key step to ensure that the spark plugs function correctly upon installation.

The other options, while important for overall vehicle maintenance, do not directly relate to the preparation required for spark plug installation. Checking fuel pressure is more about ensuring that the fuel system is functioning properly, inspecting the catalytic converter relates to exhaust emissions and system efficiency, and measuring oil levels pertains to the lubrication system, none of which involve the direct preparation of the spark plugs themselves.