The correct answer is b. They all create a courtesy bias within the customer.

When a customer sees an example, anecdote, single case, or testimonial, this would make the customer feel at least some inclination to agree with the advertisement out of politeness. These do not necessarily serve as valid indicators of the authenticity of the product or service, as they can be faked. Answer c. is wrong because they do tell us some information about what is true in general, but they do not tell us everything about the general cases, only giving some information from which we can infer about the rest. They may be but do not all have to be invalid due to the Barnum effect.