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The correct symbol for the hydroxyl ion is OH-. In chemistry, the hydroxyl ion is a negatively charged molecule consisting of one oxygen atom and one hydrogen atom. It is written as OH- to indicate the presence of an extra electron, giving it a negative charge. Options A and B (HO3 and HO4) are incorrect because they do not represent the composition of the hydroxyl ion. Option C (OH) is close but it lacks the negative charge, which is essential to signify the hydroxyl ion. Therefore, the correct answer is OH-.