Final answer:
Slippage in one gear range of an automatic transmission is most likely due to a defective or leaking clutch apply piston seal. This causes a loss of hydraulic pressure in the specific gear's clutch pack. Other issues like a defective governor, worn pump gears, or low fluid would usually impact multiple gears.
Step-by-step explanation:
Slippage in only one gear range of an automatic transmission could most likely be caused by a defective or leaking clutch apply piston seal. This seal is responsible for maintaining hydraulic pressure necessary to engage the clutch packs that control gear changes. If it's leaking or damaged, it can cause a loss of pressure that will affect only the gear that relies on that particular clutch pack, leading to slippage. A defective governor, worn transmission oil pump gears, or a low transmission fluid level could cause issues as well, but these would typically affect the transmission in more than just one gear range.