Which of the following pairs of protists and their ecological roles are correctly matched?

a) apicomplexans—parasites of animals
b) euglenozoans—primarily mixotrophic
c) dinoflagellates—parasites of plants
d) entamoebas—free-living soil organisms

The correct pair is A: "apicomplexans—parasites of animals"

Explanation:

Euglenophyta is a group of unicellular, eukaryotic organisms. They are small, free-living forms, or parasites that present different feeding mechanisms and behaviors, such as heterotrophy, autotrophy, or mixotrophy. Dinoflagellates are unicellular, flagellated, free-living protists that might form colonies. Most of them are autotrophic organisms but some of them are heterotrophic, or mixotrophic. In these last cases, dinoflagellates can feed on other dinoflagellates, protozoans, or diatoms. They can also be parasites. Entamoebas are endoparasitic organisms with no mitochondria as an adaptation of living in environments with low oxygen concentration. Apicomplexa is a unicellular, protist group. They have medical and economic importance as they are animals and human parasites. They have an apical complex that helps them to fixate to the host cell and release a substance that provokes an invagination in the host membrane. This invagination allows the parasite to get into the host cell.