23        C

Option A: F_E is outward. F_B is inward. If the two forces cancel out, net force will be zero. This allows the ion to move along a straight line at a constant speed.

Option B: F_E is in the downward and rightward direction. F_B is upward. The vertical components cancel out, net force will be rightward. This allows the ion to move straight rightward at increasing speed.

Option C: F_E is inward. F_B is downward. The net force will be inward and downward. It is not possible for the ion to travel along a straight line.

Option D: F_E is rightward. F_B is zero. The net force will be rightward. This allows the ion to move straight rightward at increasing speed.

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