Of whose death is Frankenstein informed upon returning from his trip?
This question is a little vague as it could be referring to Victor returning home from the University of Ingolstadt in Germany, or it could mean when he returns to Ireland after spending time working in a laboratory on the Orkney Islands.
If you are referring to the first instance, when Victor leaves school, he actually learns about the death of his brother William before he arrives home. It is the letter containing news of William's death that acts as the catalyst for making Victor return. The reader learns later that William was strangled by the creature Victor created; however, the murder is instead blamed on William's nanny, who is in turn sentenced to death.
If you are talking about Victor's return to Ireland, then it is Henry Clerval who is discovered dead. Henry was Victor's close friend and traveling companion, but they had separated while in Scotland so Victor could resume his work secretly. Like William, Henry was also murdered by the creature, but this time, the creature made sure that Victor would be blamed for the death. The court finds him innocent and he returns home to Geneva with his father.