Asingle atom of an element has 21 neutrons, 20 electrons, and 20 protons. which element is it?

Calcium is the element containing 21 neutrons, 20 electrons, and 20 protons.

Hope this helps

the correct option is c.

isotopes refers to different forms of the same element with the same atomic number but different mass number as a result of difference in their neutron numbers. isotopes have different relative atomic mass but share the same chemical properties.

i'm assuming that the correct answer is indeed the first one/ "a"..