He left home in 2000 and has not been heard of (yet - since - still - already).
Hi, Sedo: Ahmed_btm is right that "since" is correct. "Yet" and "still" would work in contexts in which he was expected to get in touch after he left. ("Still" would work better before "has" in that case.) But "since" is the best default choice. No one has heard from him since he left home, in 2000.
Please note that "heard of" should be changed to "heard from." If this is on an Egyptian exam, you might want to inform the test makers about their error. When we hear of someone, we acquire a little information about that person. When we hear from someone, that person contacts us.
We use "heard of [someone]" in contexts like this. Suppose I ask you if you are familiar with Philip Roth. You might say that you don't know much about him, but that you have heard of him. That is different from saying you have heard from him, which would imply that he contacted you.
- He left home in 2000 and has not been heard from since.