**Core Concept:** The process of respiration involves inhalation of air into the lungs, where the air passes through the nasal cavity, pharynx, larynx, trachea, and bronchi to reach the alveoli, where gas exchange occurs. Dust particles are inhaled during respiration and can potentially affect the respiratory system.

**Why the Correct Answer is Right:** The correct answer refers to the size of particles that can reach the alveoli, which are the tiny air sacs in the lungs responsible for gas exchange. Alveoli are surrounded by capillaries, allowing oxygen to diffuse into the bloodstream and carbon dioxide to diffuse out of the bloodstream. The primary purpose of the respiratory system is to maintain oxygenation and carbon dioxide removal from the bloodstream.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because the size of the dust particles is not directly related to the respiratory system's function. Instead, it focuses on the size of the particles, not their physiological impact.

B. Similar to option A, this option incorrectly focuses on the size of the dust particles rather than their ability to affect the respiratory system.

C. This option is wrong because the size of the dust particles reaching the alveoli is not directly related to the respiratory system's function. Instead, it discusses the number of alveoli, which is unrelated to the size of the inhaled particles.

D. This option is incorrect because it mentions the size of the dust particles reaching the alveoli, which is relevant to the question but does not address the physiological impact on the respiratory system.

**Clinical Pearl:** In clinical contexts, understanding the size of inhaled particles is essential for assessing respiratory health and potential complications. Particles larger than 5 micrometers (μm) are generally trapped in the airways, while particles smaller than 5 μm can reach the alveoli. Inhaled particles can cause various respiratory diseases, such as asthma, chronic obstructive pulmonary disease (COPD), and lung cancer. Smaller particles can penetrate deeper into the lungs and cause inflammation, tissue damage, and fibrosis, leading to these diseases.

In summary, understanding the size of inhaled particles is crucial for assessing respiratory health risks and diagnosing respiratory disorders. The correct answer (D) focuses on the size of the inhaled particles reaching the alveoli, which is relevant to respiratory health and disease.