The answer marked as correct assumes that nobody can have same tenure as Donald, but this assumption is not made explicitly in the question. If person right before Donald has same tenure, then there are MORE people with a greater tenure than Donald
The link you shared just re-directs to gregmat.com, can you re-link or share the screenshot of the problem?
I posted the same question regarding this particular problem yesterday, not knowing that you’d already raised created this post. I had the same idea as yours and thought that the 4th answer choice (“the relationship cannot be determined”) would be the correct one.