WH Questions
Excuse me, why the answer to number 10 is 'A'?
This is a common error made by Indonesian IELTS candidates, who translate directly the question form as it used in Bahasa Indonesia.
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In English you need:
Excuse me, why is the answer to number 10 'A'?
Notice that the verb follows immediately after the question word.
It might be an idea to go back to basics:
Positive
The answer to number 10 is 'A'.
Negative
The answer to number 10 is not 'A'.
Yes/No question
Is the answer to number 10 'A'?
Why question
Why is the answer to number 10 'A'?
Negative why question
Why isn't the answer to number 10 'B'?
Note that the same rule applies to other question words - what, which, where, who, how, when.