"Diez is a member of a group of immigrants who are in the desert north of Mexico in order to cross the border. Diez, just like the other immigrants, suffered from the adverse climatic conditions of that area. Excessive heat and dehydration even killed one of the woman, in the group, and almost killed him. Before that situation, Diez tries to support and encourage the group not to look at adverse circumstances and move on to their main goal, a better life. He tries to guide them, and protect them from getting caught.
The sentence Mandie congratulated win the award for the best class average makes no sense. So, the sentence Mandie congratulated Sophie on the award for the best class average is the best answer.