He took a rest from the company because it was a severe sickness. [Syntax]

The correct answer is "な". This sentence uses the causal pattern "~なので" (because ~). When a noun, like "病気(びょうき)" (illness), or a な-adjective connects to "ので", it requires "な" between them to form a grammatically correct phrase. Thus, "ひどい病気(びょうき)なので" correctly means "because of a severe illness". "ひどい" is an i-adjective modifying "病気" and is already present. "病気" is a noun already present. "ので、" is the causal conjunction itself and comes after the blank.

正解は「な」です。「〜なので」の形は、名詞やな形容詞に接続する場合「な」が必要です。「病気」は名詞なので「病気なので」となります。「ひどい」と「病気」はすでに文中にあり、「ので、」は「な」の後ろにくる接続詞のため不適当です。