Final answer:
Dinoflagellates are the protists among the options provided that are believed to have evolved following a secondary endosymbiosis. They gained their plastids through secondary endosymbiosis of red algae.
Step-by-step explanation:
Among the options provided, protists that are believed to have evolved following a secondary endosymbiosis are the dinoflagellates. During secondary endosymbiosis, one eukaryote cell engulfs another eukaryotic cell that already contains a photosynthetic endosymbiont, such as a red or green alga. Specifically, dinoflagellates are known to have acquired their plastids through the secondary endosymbiosis of red algae. This evolutionary process has endowed dinoflagellates with the ability to photosynthesize, although some can also be heterotrophic or mixotrophic, displaying extensive morphological diversity.
Other protists seen in the choices such as amoeba, Euglena, and Plasmodium do not fit the description of protists that evolved through secondary endosymbiosis. For example, the chloroplasts of euglenid protists like Euglena come from the secondary endosymbiosis of green algae rather than red algae, which is a separate evolutionary pathway.