as clear as day ：火を見るより明らか
as plain as daylight, perfectly clear/abvious というのも同意です。

It's as clear as day that I need to have a lot of writing practice in English.(Ｐｏｔａｔｏ☆Ｍｏｍの同日付の日記より)

私が英語で書く練習を沢山しなければならないのは火を見るより明らかだ。