"The elephant is his friend."
Translation:Elefanten är hans vän.
It's since been added as an accepted answer, but kompis really means more like 'pal' or 'mate'.
No, that only works if you’ve mentioned the person before in the same clause, but now the ”he” in this sentence is completely new information, so you have to use ”hans”.
No, it has to be within the same clause as well: ”Han gav sin elefant ett äpple” would work for example.
vän is the suggested answer. kompis is a more colloquial word that means 'mate' or 'pal'. It's an accepted answer although it's a less good translation, and any accepted answer can get shown to you.