✨ คำตอบที่ดีที่สุด ✨
(1) whom ← I want to talk to her(=the girl) ※to her ⇒ to whom
(2) which ← she cut the bread with it(=the knife) ※it ⇒ which(目的格)
(3) who(m) ← I go to school with him(=a boy) ※him ⇒ who(m) who/whomどちらでもいい
(4) who(m) ← you had spoken of him(=the man) ※him ⇒ who(m) who/whomどちらでもいい
(5) which ← Yumi was talking about it(=the book) ※it ⇒ which(目的格)
以上です。