Question

¿Te gusta el nuevo portátil?

No, prefiero la vieja.

No, prefiero los viejos.

No, prefiero el viejo.

Answer 1

What you need to see is the number (singular/plural) and gender (masculine/feminine) of the noun.
el = masculine singular
nuevo = masculine singular
portátil = masculine singular because both words modifying it are.

Now, what MUST you choose from the 3 choices?

I'll be back to check what you choose.

Sra