Tiene - can be stated with or without tu, so this question should be marked correct!
The pronoun can be present or omitted. We generally drop it unless it is necessary to emphasize it for some reason, i.e. clarification. It would depend on the type of exam, but I would say it shouldn't be marked incorrect.
The reason why you may have been marked wrong is because of the ending of the verb, so, if you are using "tú" you need "tienes"; if you use "usted" you need "tiene".
I checked all the questions related to this lesson and they all accept the pronoun as also correct. If you tell me which question it is you're referring to I can have a closer look for you.
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