💡 Detailed Explanation
The correct choice is 'こわされました' (was broken), which is the past polite passive form of 'こわす' (to break). The sentence structure 'A は B に [Object] を [Passive Verb]' represents adversity passive (indirect/suffering passive), where A is negatively affected by B's action. Sentence translation: 'My toy that I was treasuring was broken by my younger brother.' The other choices are: 'こわしました' (active: broke), 'こわさせました' (causative: made him break), and 'こわしてもらいました' (benefactive: had him break for my benefit, which is highly unnatural here).