The answer is C.
It is not difficult to understand why force $F$ decreases as the small metal ball is raised to the top of the sphere. The normal reaction $R$ from the hemisphere bears a greater share of the weight of the metal ball.
But why is the normal reaction $R$ the same for all positions of the ball on the sphere? Is there an intuitive explanation?
See discussion in Problem Solving Strategies chatroom on Physics Stack Exchange.