Why is plain water not suitable for thinning oil paint?
The governing concept is the compatibility of a painting medium with its thinner. Oil paint is made with an oil-based binder, whereas plain water is polar and does not combine properly with that binder. Therefore, water cannot thin oil paint into a smooth, stable mixture, so option B is correct. Options A, C, and D do not describe the actual material property involved; an appropriate oil-compatible medium is used instead.