If an element has 10 protons and 11 neutrons and 10 electrons, which expression correctly identifies the element?

When the number of protons and electrons are equal, that means the element is neutral. So, to know the element, find the element with an atomic number of 10. That would be Neon. Next, solve the mass number.

Mass number = protons +neutrons = 10+11 = 21

Thus, the expression should be written as shown in the attached picture.