Final answer:
The conjugate base of HSO4- is the sulfate ion, SO4^2-, which is formed when HSO4- donates a proton.
Step-by-step explanation:
The question is asking for the formula of the conjugate base of the hydrogen sulfate ion, HSO4−. When HSO4− donates a proton (H+), it forms its conjugate base, which in this case is the sulfate ion (SO42−). This is because by losing a proton, the charge on the ion is reduced by one. Consequently, the sulfate ion, with a formula of SO42−, is a conjugate base of HSO4− - option 3 from the multiple-choice question provided.