What causes gum deposits that can clog the carburetor and fuel system parts?

Old fuel is the correct answer because unused fuel can undergo chemical changes over time, especially when it sits in a fuel system for an extended period. These changes can lead to the formation of varnish and gum deposits, which can then clog the carburetor and other components of the fuel system. When gasoline degrades, it can lose its volatile compounds and form sticky residues that build up and interfere with the flow of fuel, ultimately leading to performance issues in small engines.

The other options, while they can influence engine performance and fuel system health, do not directly lead to the formation of the gum deposits associated with stale or old fuel. Excessive humidity can cause water to mix with fuel, potentially leading to corrosion or other issues, but it is not the primary cause of gum deposits. High temperatures can exacerbate fuel degradation but are not the fundamental reason for deposit formation as old fuel is. Overuse of additives may lead to some complications in fuel systems, but it is again not the direct cause of the gum that comes from stale fuel.