Which breaker is the trip device?

From the perspective of Panel PL2D in this image, what is the trip device?

What I believe is the "C" written in Red serial killer font. 

(My penmanship is also terrible.)

The 150A is the breaker device will trip on a fault associated with the the feeder running to panel PL2D, or a fault within the Panel PL2D bus.  This type of panel is called a "MLO" (main lug only) as it does not have a breaker inside the panel to receive the incoming cables.  An MLO panel is protected by the upstream breaker.

For Extra Credit:

The 200A breaker "B" is protecting Panel PL2A, however also serves to protect the secondary conductors of transformer TX-5.  It may also serve indirectly as protection to the transformer TX-5 depending on the transformer rating.

In regards to which breaker will actually open during a fault, its possible both the 200A and 150A breakers will open due to the similar sizes of the breakers and likely similar trip curves.  They will both see the fault current, however cannot operate quick enough to clear the fault.  Both may reach the critical point where the internal tripping system is engaged even though one of the breakers is already in the process of trying to clear the fault.  These "race to trip" scenarios are naturally undesirable and the point of coordination studies to ensure the equipment is selected and sized such that their is a clear and defined tripping scheme among the system.