Final answer:
Among the given options, HI (hydrogen iodide) is considered an acid because it releases hydrogen ions when in aqueous solution, displaying the characteristic behavior of an acid.
Step-by-step explanation:
The compound that is considered an acid among the options provided is d) HI, which is hydrogen iodide. This compound releases hydrogen ions (H+) when dissolved in water, exhibiting acidic behavior according to the Brønsted-Lowry acid-base theory. In contrast, NaOH (option a) and KOH (option c) are bases as they produce hydroxide ions (OH−) in solution, and NH3 (option b) is a weak base as it can accept H+ ions.