Which terms represents the fixed proportion of elements in a compound

Protons

Explanation:

I'm not exactly sure if this is what your question, but the number of protons is what makes an atom that atom. If you change the number of proton it is now a different atom. You change the number of electrons and get an ion or change the number of neutrons and get an isotope but you can't change the number of protons as you would be changing the elemnt.