What is an element?

a. A substance that is made of water.
b. A substance made of only one type of atom.
c. A substance that is made of no atoms.
d. A substance made of more than one type of atom.

b. a substance made of only one type of atom

Explanation:

Elements are what you find on the periodic table of elements.

Take oxygen, for example. You can have one or multiple atoms, but if they are all oxygen atoms it's an element.

"a" is incorrect because water is not an element, it is a compound.

"c" is incorrect because such a thing (as far as we know) does not exist, unless we are talking about subatomic particles like protons, neutrons, and electrons that make up an atom.

"d" is incorrect because it describes a compound.

all eukaryotic cell have mitochondria : )