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.

The correct answer is option D, All of the above

Some of the common characteristics of an index fossil are -

a) These are plants and animals preserved in the earth which are characteristics of a particular span of geologic time or environment

b) These must be different or easily recognizable

c) Must be abundant

d) Must have a wide geographic distribution

e) Must have a short range through time.

the answer is d.) homologous; have mammary glands

radon is correct hope this