Final answer:
Crackles heard on lung auscultation typically indicate that the alveoli are fluid-filled, often due to conditions like pneumonia or other respiratory diseases that alter lung compliance or resistance.
Step-by-step explanation:
Crackles heard on lung auscultation indicate fluid-filled alveoli. This sound is created by the popping open of small airways and alveoli that are collapsed by fluid, exudate, or lack of aeration during expiration and snap open during inspiration. Crackles are often associated with diseases such as pneumonia, where the alveoli become inflamed and fill with fluid, and with other conditions that cause an alteration in the compliance or resistance of the lungs, as seen in restrictive lung diseases like fibrosis or increased resistance like asthma or emphysema.
Therefore, the correct answer to the question of what crackles heard on lung auscultation indicate is D. Fluid-filled alveoli.