Which of the following is the chemical formula for the conjugate base of perchloric acid (HClO_4)?

Remove one proton (\(\mathrm{H^+}\)) from the acid formula \(\mathrm{HClO_4}\), which leaves the species \(\mathrm{ClO_4^-}\).

Recognize that the charge on the conjugate base increases by one negative charge because the proton removed was positively charged, so the conjugate base carries a negative charge.