Answer:
C. A substance with only one type of atom
Step-by-step explanation:
An element is a type of substance which is made up of only one type of an atom. Taking the example the hydrogen is made up of 1 proton and 1 electron. If any one look at the atom of hydrogen then he notice that the most of the atoms of hydrogen do not contain any neutrons but few of them contain 1 or 2 neutrons.
These different type of hydrogen is known as isotopes which contains same no. of protons but different no. of neutrons.
If you change the proton which means you change the element but if you change the neutron no. means you change its isotope.