"What do you want to buy at the supermarket?"
Translation:¿Qué quieres comprar en el supermercado?
Is it not possible to use "al supermercado"? It asked for "at the supermarket", not "in the supermarket"
"En" can also mean "at." But yes, I also wonder whether "a" could be used as an alternative for this sentence?
I wouldn't think so. "a" means "to" with the connotation of moving towards the supermarket. Buying something at the supermarket suggests you are already there.
Yo quiero ir al supermercado. I want to go to the supermarket.
Yo quiero comprar manzanas en el supermercado. I want to buy apples at the supermarket.
Is the reason that 'que' is used here instead of 'cual' that you're not asking the person to choose between options?
Yes, it is. I found this blog post helpful in explaining the difference between que and cual: http://www.onetoonespanish.co.uk/blog/the-difference-between-que-and-cual-in-spanish.htm
The preposition 'a' is used when we aim towards some place. In this sentence we are inside the supermarket where we do the shopping. Examples - ¿Quándo quieres ir al supermercado? ¿Qué quieres comprar en el supermercado? When do you want to go to the supermarket? What do you want to buy at the supermarket?
I am very confused about: ...en el.. en la/los.. It just doesn't makes sense. I always say the opposite.
The object, supermercado, is masculine and singular. Therefore it uses el.
So there are three articles: el (masculine) , la (feminine) , los (plural)? Am I missing something? What about 'de' ?
El/La are masculine and feminine singular articles. Las and Los are the plural versions. Mixed groups get masculine.
De is a preposition, not an article. It has a lot of meanings, including "of". Prepositions are one of the more complex topics in Spanish.