To observe a full moon from the surface of Earth, the alignment of the sun (A), Earth (B), and moon (C) needs to be such that the Earth is positioned directly between the sun and the moon. In this configuration, the sun illuminates the entire face of the moon that is facing Earth, making it appear fully lit.
So, if you have multiple diagrams, look for the one where the sun is on one side, the Earth is in the middle, and the moon is on the opposite side of the Earth from the sun. That is the diagram that represents when a full moon would be observed from the Earth's surface.