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I think your answer is wrong.

The most beautiful city is very specific, since there is only one most beautiful city.
If your sentence is: Paris is a beautiful city, then you can use "a". Because beautiful city is not specific, there can be hundreds of beautiful city around the world.

"where you can see the famous Eiffel Tower." is correct, because the reference to Eiffel Tower is specific, there is only one Eiffel Tower in the world.

So, your statement should be: Paris is the most beautiful city, where you can see the famous Eiffel Tower.