Answer: "Quiero comprar una falda de marca para la fiesta. Voy al mall para comprarla."
Explanation: We want to complete the sentence: 
" Quiero comprar una falda de marca para la fiesta. Voy _____________ para comprarla." 
Wich can be translated to: 
"I want to buy a skirt of a brand for the party. I will go to______ to buy it" 
the options are: 
-Al supermercado; that means "at the supermarket", you most likely will not find clothes of a good brand in a supermarket, so we can discard this option. 
-Al mall, this means "at the shopping mall", here you may find a skirt of a good brand, so this is a good option. 
- Al mercado, this means "at the market", so this option can be discarder. 
- A la verduleria, this means "at the greengrocery", so we can also discard this option. 
The only one that makes sense is the second option, "al mall" 
so the complete sentence would be: 
" Quiero comprar una falda de marca para la fiesta. Voy al mall para comprarla."