The correct option is
A The legs of the table
In the sentence, 'are' in 'are broken' indicates that multiple objects are broken. Thus, the sentence must consist of a plural subject.
Option A is correct. In this phrase the subject 'legs' is plural and so it justifies the breakage of multiple things. Thus, it justifies the presence of 'are'. Thus, it is the correct answer.
Option B is incorrect. 'Table's leg' indicates only one leg of the table. Thus, it is singular and does not justify the presence of 'are' in the sentence.
Option C is incorrect. In the phrase, the words 'one leg' indicates the only one leg is broken. Thus, the presence of 'are' is not justified.
Option D is incorrect. Similar to option C, 'one of the legs' indicates that only one leg was broken and so the presence of 'are' is not justified.