C

Proteins precipitate most readily at their isoelectric point; moving the pH away from that point actually increases solubility rather than causing precipitation [Shih et al., 1992, PMID 18601066]. In contrast, strong acids such as trichloroacetic acid, organic solvents like ethanol/acetone, and heavy-metal salts (e.g., zinc sulfate) are well-established agents used to precipitate proteins [Polson et al., 2003, PMID 12554139].