Si esta hablando con un "usted" es formal. Como maetros y proffesores. Cuando uses "tu" es familiar. Un ejemplo es con amigos y familia.
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?
<em>Where</em> are you doing at the mall?
<em>Which</em> are you doing at the mall?
<em>Why</em> are you doing at the mall?
**<em>What</em> are you doing at the mall?**
