Though there are some exceptions, most adult Olympic athletes are hardly neophytes at the sports in which they compete;
they likely enjoyed the same activities as a child.A)
they likely enjoyed the same activities
as a child.
Wrong. "they" + "as a child" ==> Not parallel.
B)
they enjoyed the same activities
as a child.Wrong. "they" + "as a child" ==> Not parallel.
In addition, B omits "likely" ==> changes meaning.
C)
they likely enjoyed the same activities
as children.
Correct. "they" + "children" ==> Keep parallel structure.
D)
he or she likely enjoyed the same activities as a child.
Wrong. The main subject of the preceding clause is "Most athletes" ==> Plural, not singular.
E)
he or she likely enjoyed the same activities
as childrenWrong. The main subject of the preceding clause is "Most athletes" ==> Plural, not singular.
In addition, "he or she" + "as children" ==> Not parallel.
Hope it helps.