Spotted deers are endowed with bright dots, and the most popular subspecies has the largest dots. The dots cannot act as camouflages for these open land grazers, due to their prominence. There is therefore some ecological reason for such bright spots. The dots must therefore be of importance to the species as the spots must be acting as some sort of signal to the deer. Which one of the following, if true, most strongly supports the conclusion regarding a signaling function?

A. Exceptional size and vigor also mark the subspecies of deer characterized by the best-defined dots
B. Grazing animals with a coat of uniform color may be harder to spot than spotted deer in certain tall grasses.
C. A temporary change of color easily can be perceived by the other members of the species, marking a visual signal in the most prevalent species of deer.
D. Deers respond much faster to moving shapes that have dots than they do to moving shapes that are otherwise identical but without dots.
E. Deers give several kinds of vocal signals than do similar species such as horses.

Argument is saying Deers have some spots. Then it is says there could be ecological reason for the same.

Conclusion: They must be acting as a signal.

To strengthen we must specify something that proves the dotes are used as a signal.

A. Exceptional size and vigor also mark the subspecies of deer characterized by the best-defined dots : Additional Info. IRRELEVANT.
B. Grazing animals with a coat of uniform color may be harder to spot than spotted deer in certain tall grasses. Other Animals are OFS.
C. A temporary change of color easily can be perceived by the other members of the species, marking a visual signal in the most prevalent species of deer. : There is no temporary change of color . Rather bright spots are permanent.
D. Deers respond much faster to moving shapes that have dots than they do to moving shapes that are otherwise identical but without dots. : WOW. So, dots are helping in someway. Hence, supporting out conclusion.
E. Deers give several kinds of vocal signals than do similar species such as horses. : Horses are OFS.
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If you look at my explanation above, I stated that to strengthen the conclusion we need to someway tell that these spots act as a signal to Deers .

Options D clearly tells that once Deers see moving shapes with Dots, they run faster. So, I could say these dots are a signal for Deers to run faster.

I hope that makes sense,
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