Melbourne is the capital city of Australia

Step 1: Analyze the sentence structure. The sentence is a declarative statement. To form a question, an auxiliary verb is needed at the beginning.
Step 2: Evaluate the options. Option 'b' correctly uses 'Is' to form a question. Options 'a' and 'c' are statements, and option 'd' uses the plural verb 'Are' with a singular subject 'Melbourne'.
Step 3: Select the grammatically correct question.
Final Answer: b. Is Melbourne the capital city of Australia?