By mother, the room's cleaning, was made to do. [Syntax]

The correct answer is 掃除(そうじ). The sentence '母(はは)に 部屋(へや) の 掃除(そうじ) を させられた' means 'I was made to clean my room by my mother.' This is a passive causative sentence. The particle を (o) marks the object of the action. Here, the action is させる (to make someone do something), and what was made to be done was the '掃除' (cleaning) of the room. Therefore, 掃除(そうじ) (cleaning) is the noun that precedes を. '部屋' is a noun, 'の' is a particle, and 'を' is a particle, none fit this noun slot.

正解は「掃除」です。この文は「母に部屋の掃除をさせられた」となります。これは受身使役の文です。助詞「を」は動作の対象を示します。ここでは「部屋の掃除」が「させられた」対象なので、「掃除」という名詞が「を」の直前に来ます。「部屋」「の」「を」は文法的に不適切です。