Why is Venus the hottest planet?

Answer 1

It is closest to the Sun.
Venus is the hottest planet in solar system because it has a atmosphere containing nearly 96% of CO2 a gas responsible for causing greenhouse effect (if you don't know green house effect Google it) making Venus literally a oven as the heat trapped by the CO2 is not able to go out.
Venus is the hottest planet because The atmosphere consists almost entirely (96%) of carbon dioxide (CO2), a greenhouse gas responsible for the incredibly hot, greater than 400ºC conditions on Venus, surpassing the melting point of lead. The rest of the atmosphere is comprised of 3% nitrogen, 0.003% water vapor, and small quantities of other gases.

Answer 2

It is very very very close to the Sun.

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Although Mercury is the closest planet to the sun, it is only the second hottest planet after Venus. Venus gets hotter as the thick carbon Dioxide atmosphere holds the heat more effectively, while Mercury has no atmosphere.