Based on the description of the graph, to estimate the amount of sunlight visible at a depth of 50 meters, one would look for the point on the graph that corresponds to 50 meters on the depth axis.
If the graph shows that at this depth, the percentage of sunlight remaining is around 0%, then the correct response would be:
0%
However, if it indicates a different percentage (like 2%, 5%, or 10%), you should select that accordingly. Please refer to the graph directly for precision.