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This is a simple one mate. The answer is a). There are only two forces acting on the string at the topmost point. The common misconception here is that people consider centripetal force to be another force acting on the particle. See, centripetal force is simply the resultant of the forces already acting, in the direction of the radius vector of the path described by the particle. For a particle to move in a circle, there must be some force holding it, just like how the merry go round cars are held together by steel rods.
Centripetal force never acts by itself, ie, it is NOT a force by itself. It is simply the resultant of the forces already acting on the body, which is directed towards the radius vector. So, at the topmost point, the only forces acting are Tension T and mg, whose resultant has to be T+mg in the downward direction.
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Moderation Team
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