Hello!
Answer:
Aux
Step-by-step explanation:
"Magasin" is a masculine word. It is accompanied by an addition of the name "de sport". We say "le magasin" since it is masculine-singular.
But since in the sentence the word "magasin" is plural, we say "les magasins".
"Les magasins" is a place, when we find the verb "être" + a place, in an affirmative sentence we say: subject + verb "être" + "à" + place.
So the sentence would be:
"Nous sommes à les magasins de sport."
But "à les" is not said in French and is replaced by the contraction "aux".
So the sentence is:
"Nous sommes aux magasins de sport."
Conclusion:
Nous sommes aux magasins de sport.