The correct answer is $\boxed{0.577 R}$.
The point velocity is the velocity at a particular point in a fluid, while the average velocity is the average of the velocities of all the points in a fluid. In turbulent flow, the point velocity is equal to the average velocity at a distance of $0.577 R$ from the pipe boundary. This is because the turbulent eddies are of a size comparable to the pipe radius, and the point velocity is equal to the average velocity at a distance of one-half the eddy size from the pipe boundary.
Option A is incorrect because the point velocity is not equal to the average velocity at a distance of $0.223 R$ from the pipe boundary. Option B is incorrect because the point velocity is not equal to the average velocity at a distance of $0.423 R$ from the pipe boundary. Option C is incorrect because the point velocity is not equal to the average velocity at a distance of $0.707 R$ from the pipe boundary.