A. Quiscalus mexicanus and Carpodacus mexicanus
B. Sylvilagus floridanus and Sylvilagus palustris
C. Procyon lotor and Zapus hudsonius
D. Enhydra lutris and Martes pennanti
The most closely related organisms will have the same genus, which is the first part of an organism's name. Since both organisms in option B start with Sylvilagus, they are most closely related.