Final answer:
The increase in heat-absorbing gases like CO2 and methane leads to the enhanced greenhouse effect, which causes global warming by trapping more heat in the Earth's atmosphere. Therefore, the correct option is A.
Step-by-step explanation:
When heat-absorbing gases such as CO2 and methane (CH4) increase in concentration in the Earth's atmosphere, it leads to a phenomenon known as the enhanced greenhouse effect. The greenhouse effect is a natural feature of Earth's atmosphere that occurs when greenhouse gases absorb and re-radiate heat back down to Earth's surface, thus maintaining a warmer surface temperature than what it would be without the presence of these gases. An increase in these gases, largely due to human activities such as burning fossil fuels and deforestation, enhances the greenhouse effect, which in turn leads to an increase in the Earth's average temperatures, a phenomenon known as global warming.