This question seems to be incomplete. However, there´s enough information to find the right answer.
Answer: Holm had a stiffer Achilles tendon that helped him jump to greater heights.
Step-by-step explanation:
In the article "The Sports Gene," by David Epstein, the inference about the jumping success of Stefan Holm that is best supported is that it was Holm´s stiffer Achilles tendon that helped him jump to greater heights.
It´s not inferred that both Holm and Donald Thomas have an exceptionally long Achilles tendon. Nor that Holm was unable to improve the height of his jumps after years of practice.