4. what must be true about an index fossil whose absolute age is used as a reference for other fossils?
the absolute age of the index fossil is actually unknown.
the absolute age of the index fossil was determined only by relative dating.
the absolute age of the index fossil was determined by radioactive dating or another absolute method.
the index fossils must be the same age as the other fossils.

The correct answer is - The absolute age of the index fossil was determined by radioactive dating or another absolute method.

The index fossils are fossils that are widespread. They are also fossils from animals that have been pretty common in certain period of the geological time. Because of this, these fossils are often used as time indicators when they are found in a same layer with other fossils, thus the name index fossils.

Still, in order for this method to be accurate, the index fossils are first examined with some of the methods that determine the absolute dating, in order to know their exact age. After their age is known, than they can be used as indicators for the age of the other fossils.