Refer to figure.
There are different types of trimming devices:

• A trim tab: which is typically installed on smaller aircraft, is another control surface that is fitted to the elevator. It's deflection can be adjusted to change the camber of the elevator itself. When moved, it changes the neutral position of the elevator.
  • The force created by the trim tab is enough to change the neutral position of the elevator, but the entire elevator range can still be used. This means that the aircraft should always remain controllable even if large control forces are required. 
• A trimmable horizontal stabiliser (THS): typically found on large aircraft including jet transport aircraft. Here, the entire horizontal stabiliser moves changing the angle of attack of the tail. This creates very strong forces, allowing the aircraft to remain in trim for a wide range of CG positions and speeds. The elevator neutral position is not changed, and full deflection of the elevator is still available. 
  • The forces created by a THS can be large enough that they overcome even full elevator deflection.

A trim runaway occurs if an autopilot and/or electric trim system is fitted, and a system malfunction causes constant uncommanded trim movement. This is quite a dangerous malfunction, and can be overcome by disconnecting the autopilot and/or the electronic trimming system. Because the forces created by a THS are so large, the effects of a stabiliser trim runaway are more serious compared to a runaway trim tab. Aircraft control may be completely lost if a runaway THS is not corrected.
Statement I → CORRECT

A jammed trim tab/stabiliser is when the trimming device becomes stuck in one position. This makes further trimming impossible. When airspeed and configuration are changed, the pilot will have to input constant control forces to maintain a given AoA. The forces due to a jammed trim tab are more easily overcome, compared to a THS. Therefore a jammed trim tab causes less control difficulty.
Statement II → CORRECT

Both of these trim malfunctions are more serious for a THS due to the much higher forces that the stabiliser can create, compared to a trim tab. Even full elevator deflection may not be enough to counteract the pitching moment created by a THS.