Which altar was constructed to celebrate the return of Augustus?

The Ara Pacis, or Altar of Peace, was constructed to commemorate the return of Augustus to Rome in 13 BC after his campaigns in Gaul and Spain. It symbolizes the peace and prosperity brought about by Augustus' reign and features intricate relief sculptures depicting mythological and historical scenes, emphasizing the themes of peace and the blessings of the gods over the Roman people.

In contrast, the Altare della Patria, known as the Altar of the Fatherland, is a monument built much later to honor King Vittorio Emanuele II, the first king of unified Italy, and is not directly related to Augustus or his return. The Temple of Mars was dedicated to the god of war and serves a completely different purpose in the religious landscape of Rome. The Altar of Janus, dedicated to the god associated with beginnings and transitions, also does not pertain to the celebration of Augustus' return.

Thus, the reasons for the creation of the Ara Pacis clearly align with Augustus' return, making it the correct choice among the options provided.