Correct answer:
buscan
Since Alejandro y tú matches the first person plural, that is, nosotros in Spanish that translates into we in English, then we must conjugate the verb buscar matching this person as well in the simple present. This conjugation is buscan. So we can write this sentence as follows:
Alejandro y tú buscan el di.ccionario (Alejandro and you are looking for the dictionary)