The commonly used thinner in oil paints, is

Thinner is a solvent added to paint to reduce its viscosity, making it easier to apply.
Let's look at the options:
Option A: Naphtha - Naphtha can be used as a solvent, but it is not as common as turpentine in oil paints.
Option B: Turpentine - Turpentine is a widely used and very effective thinner for oil paints.
Option C: Both (A) and (B) - Since Turpentine is more commonly used, this option is a possibility.
Option D: None of these - We know Turpentine is used, so this is incorrect.
Therefore, the commonly used thinner in oil paints is Turpentine, though Naphtha can also be used.
So the best answer is Option C: Both (A) and (B).