Stimuli in an arbitrary stimulus class: a. share common physical forms or relations to the target stimulus. b. do not share similar stimulus features. c. include an infinite number of stimuli. d. comprise a large portion of our conceptual behavior.

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.