Both protons and neutrons are located inside the nucleus of an atom. changing the number of neutrons in the nucleus would form different isotopes of the same element. which of these best describes the outcome of decreasing the number of protons in the nucleus?

By changing the number of protons in an element, you are fundamentally changing it to a different element.

It’s worth remembering that:

Protons identify the element

Electrons determine properties and behavior of the element

Neutrons contribute to isotopes and just help keep everything stable, by separating the charges. Remember opposites attract. Like charges repel.