Final answer:
The implied subject of the sentence is 'you,' referring to the person being addressed, which in this case is the brother. The term 'Brother' serves as a vocative expression, not a grammatical subject.
Step-by-step explanation:
The subject of the sentence "Brother darling! You're right on cue!" is not explicitly stated but implied. In English grammar, the subject is the person or thing that is doing the action or being described. However, in commands or exclamations, the subject is often implied and is usually the person being addressed, which can be deduced from context or vocative expressions. In this case, 'Brother' is used as a vocative and is not the subject of the sentence; instead, the implied subject is 'you', referring to the brother being addressed.
Considering the information provided in the reference, none of the options (A) Ursula, (B) Brother, (C) Darling, or (D) Cue, accurately identifies the implied subject of the sentence. The implied subject 'you' is not provided among the options. In a dialogue exchange like the one from the reference, Mrs Linde expresses her right to take pride in what she has done for her brothers, which might imply a context in which 'brother' is addressed with affection as 'darling', still leaving 'you' as the implied subject.