Right choice is (c) (r + R) tan φ
The best explanation: Interference may only be prevented, if the addendum circles of the two mating gears cut the
common tangent to the base circles between the points of tangency.
maximum length of arc of contact = (r + R) tan φ
where r = Pitch circle radius of pinion,
R = Pitch circle radius of driver, and
φ = Pressure angle.