Fill in the blank with suitable modal from the options below:

"Who is she? She ______ be his sister".

must

The correct answer is Option (1) → must

So the complete sentence will be: "Who is she? She must be his sister."

• Must indicates a strong assumption or conclusion based on evidence or reasoning.
• Should implies advice or recommendation, which is not suitable in this context.
• Would suggests a hypothetical situation and is not appropriate for making an assumption.
• Need is not a modal that fits this context, as it conveys necessity rather than assumption.