Which of the following sentences could take the place of:

ustedes compraron un regalo para ellos.

los lo compraron.
se lo compraron.
se les compraron.

The last one... Se les compraron
Ustedes compraron un regalo para ellos translates into You bought a gift for them..
Los lo compraron means they bought it
So does se lo compraron
But se les compraron means They were bought 

For this question, it will be important to use the words "mejor" (better), "lo mejor" (the best), and "lo peor" (the worst). while "mejor" is comparative (you are comparing one thing to something else), "lo mejor" and "lo peor" are superlative (there is nothing else better or worse than what these words are describing - there is no comparison).  you could say something like: la música rap es mejor que la música clásica - que es lo peor - pero la música rock es lo mejor de todas las formas de la música (rap music is better than classical music - which is the worst - but rock music is the best out of all the forms of music).