Which of the following can result from fuel system contamination?

Fuel system contamination primarily refers to the presence of foreign particles or substances in the fuel, which can significantly impair the proper functioning of the fuel system and, consequently, the engine. When the fuel system is contaminated, it can cause various issues, one of the most notable being engine roughness. This is because contaminants can disrupt the fuel flow, cause uneven combustion, or lead to blockages in the fuel lines and filters. As a result, the engine experiences irregular performance, which can manifest as rough running, misfires, or stalling.

When considering the other options, it's evident that they depict positive outcomes that are generally contrary to the effects of contamination. Increased altitude performance, fuel efficiency improvement, and enhanced pilot control imply an optimal functioning fuel system and engine performance, which cannot be achieved when contamination is present. Contamiantion typically leads to engine inefficiencies and performance issues rather than improvements. Thus, the occurrence of engine roughness is a direct and expected consequence of fuel system contamination.