High engine idle speed will likely cause which of the following?

High engine idle speed can lead to low line pressure and early upshifts due to the relationship between engine speed and transmission operation. When the engine idles at a higher speed than normal, it can interfere with how the transmission's hydraulic system operates. The line pressure is crucial for engaging clutches and bands appropriately, which control gear shifts in an automatic transmission. If the line pressure is lowered by the elevated idle speed, it can cause premature upshifts, meaning the transmission shifts to a higher gear sooner than it should, potentially leading to poor performance and drivability.

In this context, while harsh engagement from neutral to drive and direct upshifts from first to third are possible effects in an automatic transmission system, they are more closely associated with different operational issues. High idle speeds primarily influence line pressure and shift timing, making this the most accurate consequence among the options presented.