—¿ haces en el centro comercial? —voy de compras. qué por qué cuál dónde

Let's look at the meanings of each word to find the best

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?**