I managed to obtain a ticket for the opera,but ---- at the b

选A
后面的,but --- at the back of the theatre,I coule neither see nor hear clearly.是一个复合句,前面的but at the back of the theatre是过去分词短语表示原因,所以这里相当于在but后面有个be.
而四个选项中可以和be搭配连接的只有seated
be seated表示坐下,和sit down意思差不多