Final answer:
The planet characterized by a dense CO₂ atmosphere, retrograde rotation, and a runaway greenhouse effect is Venus. Its atmosphere is composed of 96% CO₂ and has led to extremely high surface temperatures. The correct option is b.
Step-by-step explanation:
The planet with a dense CO₂ atmosphere, retrograde rotation, and a runaway greenhouse effect is Venus. Venus has an atmosphere that is 96% CO₂ with a significant greenhouse effect maintaining extreme surface temperatures due to the thermal energy trapped by this thick CO₂ layer.
Venus also rotates in a direction opposite to most other planets in our solar system, a movement known as retrograde rotation, which has a rotation period of 243 days. This means that Venus spins from east to west, opposite to the direction of Earth and most other planets. The combination of these factors, along with the loss of H₂O from its atmosphere, has led to Venus' current inhospitable conditions.
In comparison, Earth has liquid water and supports life thanks to its balanced greenhouse effect, which is mostly from water vapor and carbon dioxide, providing an average surface temperature increase of about 33 °C. Mars, with its thin atmosphere and very little greenhouse warming, is much colder, indicating that it must have been different in the past. The runaway greenhouse effect has made Venus an extreme example of the consequences of a thick CO₂ atmosphere compared to these other terrestrial planets.
Hence, Option b is correct.