Which organism would be the best index fossil? ammonite
shark
O trilobite
O brachiopod

B. Centrosome

Explanation:

Centrosomes are where microtubules are organized, thus it's called the microtubule-organizing center. They are made up of centrioles that help in forming microtubules. Centromeres help in changing the shape of the cell membrane allowing it to divide into to two. It also helps in organizing the chromosomes during cell division.