The element's number of electrons is determined by the atomic number.
Silicon has an atomic mass of 28.09. Round it up, the whole number is 28.
The atom must have 14 neutrons AND 14 protons with a mass that adds up to 28.
So, the ones correct are:
The atom must have 14 protons, neutrons, and electrons.
(Note: This is correct if you want, like, I don't know, the appropriate standard status for Silicon.)
The "The atom must have 14 protons and electrons, but can have a different number of neutrons." is true, but please note that would make Silicon an isotope. Isotopes are atoms of the same element, but different masses. That implies it would have a different amount of neutrons. The number of protons is always the atomic number. In other words, the protons stays the same.
This is false:
"The atom must have 14 electrons, but can have a different number of protons and neutrons." is false because if you f with the protons, it's no longer silicon. You feel me?
"The atom must have 14 neutrons." is false because the number of neutrons does not determine the atom's identity. The protons determine the identity. So Siliicon is still silicon if it has 15 neutrons, it's still Silicon. But if it had 15 protons, it's no longer Silicon. It would be the next element after it, Phosphorus.
Hope this helps! Sorry for the wait. :(
