A. Caucasian workers had; by 2013, they were earning almost six dollars less
"they" is referring back to Hispanic workers. parallelism looks okay.. hold on to this one.
B. Caucasian workers had earned, a figure that was almost six dollars less by 2013
not so fond of this but hold on.
C. Caucasian workers did; this figure had decreased to almost six dollars by 2013
after semicolon should be subject ,not "this figure". incorrect
D. Caucasian workers, which was almost six dollars less by 2013
"which" referring back to closest noun ruins it. incorrect
E. had Caucasian workers, a figure that had decreased to almost six dollars less by 2013
parallelism error. "hispanic workers....than had" incorrect
between A &B, in B, "earned" is repeated which makes it redundant ,however noun modifier works fine.
in A, earning six dollar less. less than previous figure or less than the actual earnings ??
IMO B.