Since the particle is just able to complete the circle, at the highest point only its weight keeps it in circular path and tension in the string will be zero, hence option A is right. In case of option B, the particle completes a full circle. Therefore, at the highest point, it must have horizontal velocity equal to .

Option C is also not true because initial kinetic energy should be .

Since no further work is done on the particle by an external force, the total mechanical energy of the systems remains constant and the particle passes the initial position with the same initial velocity. Hence option D is correct.