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.
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.