Quick Biology question. Can anyone clarify this for me, i am quite confused. I understand the question except for what they mean by a multiple of 3 or not a multiple of 3. What does it mean by multiple of 3 or what is it referring to? Thank you very much!!

Which of the following must be true for a fossil organism to be useful as an index fossil? a. the fossil organism must be widespread geographically. b. the fossil organism must be abundant. c. the fossil organism must be limited to a short span of geologic time. d. all of the above.