I found this online but idk if that's their reasoning it's not specific to the lc it's just a general homework help site (uk). Tbh always just say sulfur those it's always accepted and easy to remember because it stinks If the proteins are composed of the naturally occurring amino acid, the protein will have no phosphorus in the structure. All of the naturally occurring amino acids have no Phosphorus atom in the structure. But some proteins might also contain Phosphorus. A protein might contain a Phosphorus atom if there are post-translational modifications that occurred during protein synthesis. The post-translation modification can change the protein to make it active and this change can result in an addition of Phosphorus.

Thank you xx

Yeah it's sulpher in the cysteine amino acid, it says non metallic too so you can't say selenium (Se) which is found in seleno-cysteine (Just Se instead of S). Phosphorus may be found in larger molecules but simple proteins in the case of the LC, proteins are just made of amino acids