Gear backlash is best described as which of the following?

Correct Answer: Option D — Play between the surfaces of the teeth in mesh.

Backlash is the clearance or 'play' between the mating surfaces of gear teeth when they are in mesh. This clearance is essential to accommodate thermal expansion, provide space for lubricating oil film, and prevent the gears from binding or jamming during operation, which could lead to catastrophic gear failure.