The best version of the underlined portion of sentence 13 is "are concerned" (Answer D). This choice correctly uses the present tense to indicate that the proponents of biomedical scent detection currently have concerns about the health-care environment and its impact on the dogs and patient interactions.
So, the final sentence would read: "Even proponents of biomedical scent detection are concerned that aspects of the health-care environment might distract dogs from their tasks..."