The answer is option D "test the effect of cyanide on algae." Ecologist are researchers who study how organisms interact with their environment so in this case they are testing how algae reacts to cyanide. In another example they will be testing how a ape reacts to having a different fruit tree in their habitat. It wouldn't be option A, B, and C because you have to remember the job of a ecologist and the options are not jobs of one.
Hope this helps.