A coin tie with a string held at one end is being rotated at a constant angular velocity in the air. In what case will the tangential velocity be constant ?

a) The plane of motion of the coin is parallel to the ground
b) The plane of motion of the coin is perpendicular to the ground
c) The plane of motion of the coin is at an angle of 45 degrees to the ground
d) The tangential velocity of the coin is independent of the plane of motion

Only a

When the plane motion of the coin is parallel to the ground, the tangential velocity experiences acceleration in two directions, one towards the center and other towards the ground. Both these directions are perpendicular to the direction of the tangential velocity. Thus, the tangential velocity will not change.