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A rich air-fuel ratio, characterized by too much fuel relative to air, can indeed be influenced by various factors, including fuel pressure. High fuel pressure is a primary cause of a rich condition because it increases the amount of fuel delivered to the engine, leading to an air-fuel mixture that is richer than intended. If the fuel pressure regulator is malfunctioning or if there are restrictions in the fuel return line, the fuel pressure can rise too high, exacerbating the richness of the mixture.

Considering the other options, low fuel pressure would generally lead to a lean condition rather than a rich one, as there would not be enough fuel available for proper combustion. A bad camshaft could also impact engine performance, but it typically affects timing and valve operation rather than directly altering the air-fuel ratio to be rich. Therefore, the most direct cause of a rich air-fuel ratio in this particular context is high fuel pressure, making it the most appropriate answer.