Answer:d.) protozoa
Explanation:protozoa is a unicellular eukaryotic organism that belongs to kingdom protista. Protozoans depends on other organism for there food because they do not produce there own food. Some protozoa can be parasitic in nature.
Example of protozoa is amoeba.
The correct answer is option B and C, that is, they do not share similar stimulus characteristics and include a restricted number of stimuli.
An arbitrary stimulus class refers to predecessor stimuli, which induce the similar reaction, however, they do not share relational feature and do not resemble each other in physical form. For example, cheese, peanuts, chicken breasts, and coconut milk are the members of an arbitrary stimulus class if they induce the response, that is, sources of protein.