Let's look at the meanings of each word to find the best answer:
Qué - What
Por qué - Why (directly translates to "for what")
Cuál - Which
Dónde - Where
Now, let's look at the meaning of the sentence.
... haces en el centro comercial? = ...are you doing at the mall?
Which answer fits best in english?
Where are you doing at the mall?
Which are you doing at the mall?
Why are you doing at the mall?
**What are you doing at the mall?**
