Protist taxonomy has changed greatly in recent years as relationships have been

The protists refer to the unicellular eukaryotic species, which comprise algae, fungi, and protozoa. Of these some are autotrophs and some are heterotrophs. They exhibit the tendency to survive in aquatic surroundings because of their feeding habits.  

In older approaches, protists are categorized on the basis of their morphology. However, by using novel approaches like molecular genetics, they are being categorized on the basis of their genetics.  

The definition to parasitism is "the practice of living as a parasite in or on another organism." (Wikipedia). Therfore the answer to this is Ticks live on and drink blood from a cow, because this is the only option where an organism is living directly off another organism.

Hope I was most helpfull to you!