**Core Concept**
The movement of oxygen from the alveoli into the blood in the pulmonary capillaries is a critical process for gas exchange. This process is primarily facilitated by the difference in partial pressures of oxygen between the alveoli and the blood.

**Why the Correct Answer is Right**
The correct answer, _Diffusion_, is responsible for the movement of oxygen from the alveoli into the blood in the pulmonary capillaries. This process occurs through the process of simple diffusion, where oxygen molecules move from an area of high concentration (alveoli) to an area of low concentration (blood) across the thin alveolar-capillary membrane. The partial pressure of oxygen in the alveoli is higher than in the blood, driving the diffusion of oxygen into the blood. This process relies on the presence of a favorable concentration gradient and the permeability of the alveolar-capillary membrane.

**Why Each Wrong Option is Incorrect**
**Option A:** _Osmosis_ is incorrect because osmosis is the movement of water molecules from an area of high concentration to an area of low concentration across a semipermeable membrane. While osmosis does occur in the alveolar-capillary membrane, it is not responsible for the movement of oxygen into the blood.
**Option B:** _Active transport_ is incorrect because active transport requires energy to move molecules against a concentration gradient. The movement of oxygen into the blood in the pulmonary capillaries does not require energy, as it occurs down a concentration gradient.
**Option C:** _Oxidative phosphorylation_ is incorrect because oxidative phosphorylation is the process by which cells generate energy from the transfer of electrons during cellular respiration. While oxidative phosphorylation is an important process in cellular metabolism, it is not directly involved in the movement of oxygen into the blood in the pulmonary capillaries.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that diffusion is a critical process for gas exchange in the lungs, and any disruption to the alveolar-capillary membrane or the concentration gradient can lead to impaired oxygenation of the blood.

**Correct Answer:** A. Diffusion.