Venus
The atmosphere of Mars is almost entirely CO2.
Too much carbon dioxide in the atmosphere, as we have now, 400 ppm (parts per million) or 0.04% instead of the regular 280 ppm that we've had for thousands of years, is the cause of global warming. This is very bad for the planet.
Presuming you mean which planet has carbon dioxide as the primary component of its atmosphere, and features clouds of sulfur dioxide (or sulfuric acid), which would be Venus, second planet from the Sun (the next innermost planet from the sun compared with earth).
The planet that is half the size of the earth, has 2 moons, and has an atmosphere of mostly carbon dioxide is Mars. The average length of a day on Mars is 37 minutes longer than that of the Earth.
The main gases in Earth's atmosphere that help keep surface temperatures warm enough for life are carbon dioxide, water vapor, and methane. These gases trap heat from the sun in a process known as the greenhouse effect, which helps maintain the planet's temperature within a range suitable for life.
Venus.
Venus has a thick carbon dioxide atmosphere, making up about 96.5% of its atmosphere. This high concentration of carbon dioxide contributes to the intense greenhouse effect on the planet, resulting in surface temperatures hot enough to melt lead.
That would be Venus. Its dense carbon dioxide atmosphere traps heat using the "greenhouse effect", making the temperatures so high that the surface of the planet is soft and melted.
Venus has a dense carbon dioxide atmosphere with surface temperatures that can exceed 800 degrees Fahrenheit (427 degrees Celsius).
Venus has an atmosphere mostly of carbon dioxide and has a runaway greenhouse effect. Temperatures there are over 460 °C (860 °F).
Venus has the most carbon dioxide atmosphere of any planet in the solar system, with about 96.5% of its atmosphere composed of carbon dioxide. This thick atmosphere contributes to the planet's extreme greenhouse effect and high surface temperatures.
Venus has a dense carbon dioxide atmosphere and high surface temperatures, with an average surface temperature of about 467 degrees Celsius (872 degrees Fahrenheit). This extreme greenhouse effect is due to the trapping of heat by the carbon dioxide in the atmosphere.
Venus has a hot, turbulent atmosphere dominated by carbon dioxide. The thick atmosphere traps heat, creating surface temperatures high enough to melt lead.
Venus has an atmosphere made up of hot swirling clouds of carbon dioxide, which create an intense greenhouse effect and contribute to its extreme surface temperatures.
The planet is likely Venus, which has an atmosphere composed primarily of carbon dioxide. This high concentration of carbon dioxide contributes to a runaway greenhouse effect, resulting in extreme surface temperatures and a dense, opaque atmosphere.
Carbon dioxide (CO2) is the primary greenhouse gas in Venus' atmosphere that traps the most heat. It accounts for over 96% of the atmosphere and contributes significantly to the planet's extreme temperatures.
A thick carbon dioxide atmosphere and chronic global warming with temperatures up to 500 degrees C.