① The science of medicine is perhaps the most important of all sciences. +
② Progress has been very rapid in it(代名詞・目的格=the science of medicine)
⇒ The science of medicine, in which progress has been very rapid, is perhaps the most important of all sciences.
※②の文中にあるin it → in which(関係代名詞・目的格)に変えて一つの文にまとめたものが問題文
文の意味:進歩が非常に速い医学は、恐らく全ての科学の中で最も重要である。
参考にしてください。
訂正:②Progress has been very rapid lately in it(代名詞・目的格=the science of medicine).
⇒ The science of medicine, in which progress has been very rapid lately, is perhaps the most important of all sciences.
文の意味:最近進歩が非常に速い医学は、恐らく全ての科学の中で最も重要である。