**Core Concept**
The question is testing the understanding of the relationship between fractional inspired oxygen (FiO2) and arterial oxygen tension (PaO2) in a normal person. This concept is based on the principles of gas exchange and diffusion of oxygen from the alveoli into the blood.

**Why the Correct Answer is Right**
In a normal person, the PaO2 is directly proportional to the FiO2. When FiO2 is increased to 0.5, the alveolar oxygen tension increases, leading to an increase in the diffusion of oxygen into the blood. This results in an increase in the PaO2. According to the alveolar gas equation, PaO2 = (FiO2 x (Pb - PH2O) - (PaCO2 / R)) + (1.2 x FiO2), where Pb is the atmospheric pressure, PH2O is the partial pressure of water vapor, PaCO2 is the arterial carbon dioxide tension, and R is the respiratory quotient. Assuming a normal PaCO2 and R, the increase in FiO2 will directly increase the PaO2.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it assumes that the PaO2 will remain unchanged with an increase in FiO2, which is not true in a normal person.
**Option B:** This option is incorrect because it suggests that the PaO2 will decrease with an increase in FiO2, which is the opposite of what is expected.
**Option C:** This option is incorrect because it suggests that the PaO2 will remain unchanged with an increase in FiO2, which is not true in a normal person.

**Clinical Pearl / High-Yield Fact**
In a normal person, a FiO2 of 0.5 will increase the PaO2 by approximately 100 mmHg, assuming a normal PaCO2 and R. This is a useful estimate to keep in mind when managing patients with respiratory failure.

**Correct Answer: A. 300 mmHg**