Nave is part of the church, the basilica where the people stood during the religious service, while the narthex is the front entrance of the basilica, the front room before entering the nave. The altar was in the apse which was facing the east, opposite to the entrance of the basilica on the west side. Altar is the place where the sacred ritual was performed, in the apse of the semi-spherical shape.
The answer is: at the apse.
