Option 1 -> Fratricide means killing one's brother or sister.
Option 2 -> Matricide means killing one's mother.
Option 3 -> Patricide means killing one's father.
Option 4 -> Infanticide means killing an infant or newborn child.
Hence, Option 1: Fratricide -> The sentence specifically mentions "he has killed his brother," and fratricide is the precise term for the act of killing one's sibling (brother or sister) -> correct