One-year-olds ordinarily prefer the taste of sweet food to that of

1 yr olds prefer sweet food over salty food.
If one feeds a one-year-old salty food rather than sweet food, then at 2 yr old, the child will prefer salty over sweet.

Conclusion: A young child’s taste preferences can be affected by the type of food he or she has been exposed to.

We are concluding that the child's preferences are affected by what is fed to him/her. So we are assuming that a 2 yr old is not naturally inclined to salty over sweet. That the food fed to the child caused the change in preference.
If it were true that a 2 yr old naturally prefers salty over sweet then what food he was fed may have no relevance.

(A) Two-year-olds do not naturally prefer salty food to sweet food.

Yes, we are assuming that a 2 yr old does not naturally prefer salty over sweet. Correct

(B) A child’s taste preference usually changes between age one and age two.

Not assumed. In fact, the argument would make more sense if there were no natural changes in food preference from age 1 to 2.

(C) Two-year-olds do not naturally dislike salty food so much that they would not choose it over some other foods.

Naturally preferring salty foods is far better than "naturally not disliking salty foods". Also here we are talking about "some other foods". Option (A) clearly talks about preferring "salty food over sweet food". This is not the right option.

(D) The salty food fed to infants in order to change their taste preferences must taste pleasant.

Irrelevant. We don't know what role pleasant taste plays.

(E) Sweet food is better for infant development than is salty food.

Irrelevant. No discussion on what is better.

Answer (A)