Final answer:
The joke Eli's friend made when his number was not written down for selection is that he is invisible to them.
Step-by-step explanation:
The joke that Eli's friend made when his number was not written down for selection is option D) "You're invisible to them." This joke implies that Eli is not being noticed or recognized by the selection process. It suggests that Eli's friend found it amusing that Eli's number was not chosen, as if he was somehow not seen by the selectors.