All people prefer colors that they can distinguish easily to colors th

Isn't D kind of an assumption here, rather than being an conclusion ?

In order to have an assumption, we need a conclusion. Where is the conclusion here... ?

All people prefer colors that they can distinguish easily to colors that they have difficulty distinguishing. Infants can easily distinguish bright colors but, unlike adults, have difficulty distinguishing subtle shades. A brightly colored toy for infants sells better than the same toy in subtle shades at the same price.

Which one of the following conclusions is most strongly supported by the information in the passage?
Point is -

A brightly colored toy for infants sells better than the same toy in subtle shades at the same price.

(A) Infants prefer bright primary colors to bright secondary colors.
no mention- primary / secondary.
(B) Color is the most important factor in determining which toys an infant will prefer to play with.
strong word- Most (not stated in the passage also).
(C) Individual infants do now have strong preferences for one particular bright color over other bright colors.
no mention.
(D) The sales of toys of infants reflect the preferences of infants in at least one respect.
there might be more parameters but at least one parameter / preference of infants is taken care of.
Money is other which is buyer's preference, not infants'.
correct.
(E) Toy makers study infants to determine what colors the infants can distinguish easily.
may be. no mention.

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