**Core Concept:** The **Greenhouse Effect** is a natural process that warms the Earth's surface by allowing sunlight to pass through the Earth's atmosphere and then trapping the re-emitted infrared radiation, preventing it from escaping into space. This phenomenon is crucial for maintaining the Earth's habitable temperature.

**Why the Correct Answer is Right:** The correct answer, **D**: **Carbon dioxide (CO2)**, is part of the greenhouse gases responsible for the greenhouse effect. These gases, including water vapor, methane, nitrous oxide, and ozone, allow sunlight to pass through the atmosphere but trap infrared radiation, thus contributing to the warming of the Earth's surface.

**Why Each Wrong Option is Incorrect:**

A. **Methane (CH4)** is also a greenhouse gas, but it is not the correct answer because it is not the primary gas responsible for the greenhouse effect.
B. **Water vapor (H2O)** is the **most abundant greenhouse gas** in the Earth's atmosphere, making it the most significant contributor to the greenhouse effect.
C. **Ozone (O3)** is a crucial component of the Earth's atmosphere but is not the correct answer because it is a gas that absorbs harmful ultraviolet radiation and protects life on Earth, not causing the greenhouse effect.

**Clinical Pearl:** The **Greenhouse Effect** is essential for maintaining life on Earth, but excessive amounts of greenhouse gases, particularly CO2, can lead to **Climate Change**, a phenomenon caused by human activities such as burning fossil fuels and deforestation, which release large amounts of CO2 into the atmosphere. Climate change can result in severe consequences, including rising temperatures, extreme weather events, and loss of biodiversity.

**Correct Answer: D. Carbon dioxide (CO2)** is the correct answer as it is the primary gas contributing to the greenhouse effect and its concentration in the atmosphere has significantly increased due to human activities.