Final answer:
The parasitic protist that evades the host immune system by altering its surface proteins with each generation is Trypanosoma brucei. This process, known as antigenic variation, is a survival mechanism that helps the parasite to evade immune responses and maintain an infection. The correct option is D.
Step-by-step explanation:
The question relates to how certain parasitic protists are able to evade the host immune system. Specifically, some protists can alter their surface proteins with each generation, making it difficult for the immune system to recognize and attack them. This process is known as antigenic variation.
An example of such a protist is Trypanosoma brucei, which causes African sleeping sickness. Trypanosoma brucei is adept at changing its surface glycoproteins, helping it to evade the immune response. On the other hand, viruses like HIV can also avoid immune detection by using the host's membrane to disguise themselves.
This alongside rapid mutation alters the antigens on their surface quickly, leading to a decrease in the effectiveness of antibodies against the virus.
In the context of the question's answer choices, Trypanosoma brucei is the correct answer as it is known for its ability to change its surface proteins and evade the host's immune system. This mechanism of immune evasion highlights the complex interactions between pathogens and the immune system, and how pathogens can adapt to avoid detection and destruction.
Additionally, other mechanisms like production of capsules, utilization of host molecules to mask pathogen antigens, and outright attack on immune cells are also referred to in the question's context.