Because very was hot, water, drank. [Syntax]

The correct answer is "た". "た" is the plain past tense ending for い-adjectives. It attaches to the stem "あつかっ" to form "あつかった" (it was hot), which is then correctly followed by "から" to express a reason ("because it was hot"). This plain past tense form is crucial for setting up the reason for the action.

正解は「た」です。「た」はい形容詞（いけいようし）の普通形（ふつうけい）の過去形（かこけい）の語尾（ごび）です。「あつかった」で「暑（あつ）かった」という意味（いみ）になり、その後に理由（りゆう）を表す「から」が続きます。