I need help on how to fill this in.

the answer is d. chordata

there are so many examples for that in different areas, like biology experiment carried out in our lab recently.

here's one link:

in a chemical reaction, the atoms and molecules produced by the reaction are called products. no new atoms are created, and no atoms are destroyed. in a chemical reaction, reactants contact each other, bonds between atoms in the reactants are broken, and atoms rearrange and form new bonds to make the products.