Final answer:
The conjugate base of HClO3 is ClO3- (option B), which is formed by removing a proton from the acid in accordance with the Brønsted-Lowry Acid-Base Theory.
Step-by-step explanation:
The conjugate base of an acid is formed by the removal of a proton (H+) from the acid. Considering HClO3 (chloric acid) and applying the Brønsted-Lowry Acid-Base Theory, we identify the conjugate base by removing one hydrogen ion from the acid, leaving us with ClO3−. Thus, the correct answer to what the conjugate base of HClO3 is: B. ClO3−.