A) 3 and 4 b) 2 and 3 c) 1 and 2 d) 1 and 4

d) 1 and 4

Explanation:Elements are made up of particles known as atoms. Atoms of the same element have the same properties as the element.The atoms of an element may differ in terms of the number of neutrons and have a different mass number, such atoms are known as isotopes.However, the isotopes have an equal number of protons or atomic number.In this case, atom 1 has a mass number of 20 and an atomic number of 10 while atom 4 has a mass number of 21 and an atomic number of 21.Therefore, atom 1 and 4 are isotopes of element E.

It was ernest rutherford