**Core Concept**
The question is testing the ability to differentiate between gases in tissue, which is a critical concept in pathophysiology and clinical diagnosis. The gases in question are carbon dioxide (CO2), oxygen (O2), nitrogen (N2), and hydrogen (H2).

**Why the Correct Answer is Right**
The correct answer is related to the difference in the solubility of gases in blood and tissues. Carbon dioxide is more soluble in blood and tissues compared to oxygen, nitrogen, and hydrogen. This difference in solubility is due to the unique properties of each gas, including their molecular weight and the affinity of hemoglobin for CO2. The high solubility of CO2 in blood and tissues allows it to be efficiently transported and removed, whereas the low solubility of O2, N2, and H2 makes them less soluble in these environments.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because oxygen is less soluble in blood and tissues compared to carbon dioxide. While oxygen is essential for cellular respiration, its low solubility in blood and tissues makes it more challenging to transport and utilize.

**Option B:** Incorrect because nitrogen is an inert gas that is not metabolically active and does not have a significant impact on tissue gas levels. Its low solubility in blood and tissues is due to its molecular properties, but it is not relevant to the clinical context of gas in tissues.

**Option C:** Incorrect because hydrogen is not typically considered a gas in tissue, and its low solubility in blood and tissues is not a significant factor in clinical diagnosis or pathophysiology.

**Clinical Pearl / High-Yield Fact**
When differentiating between gases in tissue, it's essential to remember that carbon dioxide has a higher solubility in blood and tissues compared to oxygen, nitrogen, and hydrogen. This difference in solubility is critical in clinical diagnosis and pathophysiology, particularly in conditions such as respiratory acidosis or alkalosis.

**Correct Answer: D. Carbon dioxide (CO2)**