In this investigation of solar energy, the most plausible result is:
The water in container 2 will be warmer because the mirror reflects solar energy away from container 1 and toward container 2.
The reason for this is that the mirror, when placed above the table and angled correctly, will reflect sunlight onto container 2, thus increasing its temperature. On the other hand, container 1, which is situated directly below the mirror, would not receive the reflected sunlight and is likely to remain at a lower temperature compared to container 2.