Help identify electrical boxes
Basically A is a time clock for B a low voltage transformer.
C looks to be the hot in and D looks to be the hot out from the time clock to switch the low voltage transformer ( which apparently is being direct wired by cutting off the male plug).

Another scenario is the time clock does or used to do high voltage tree lights.  The low voltage transformer used to be plugged into an outlet but is not used anymore.  Just guessing.