Tú compras un boleto para ellos.
From the options, it appears you need to write this sentence using a direct object pronoun. The object of the sentence is un boleto, which is masculine singular. This means the direct object pronoun must be masculine singular; the direct object pronoun must be lo.
This narrows the options to 1 and 2, however the first option can be eliminated due to grammatical error (los lo, which translates to "the the", doesn't make sense).
Answer:
2) Se lo compras.
