Option 1 is the correct answer, indicating both that the assertion and reason are individually true and that the reason is the correct explanation for the assertion. The assertion is that human life on Venus is highly improbable, which is a plausible statement given current scientific knowledge. The reason provided is that Venus has exceptionally high levels of carbon dioxide in its atmosphere, which is also correct. High levels of carbon dioxide in the atmosphere makes it impossible for human life to exist due to the lack of oxygen and extremely high temperature caused by greenhouse effect. Thus, the reason is indeed a correct explanation of the assertion.

The other options are incorrect. Option 2 would suggest that although both statements are true, the high level of CO2 is not the reason human life is improbable on Venus. Option 3 claims that the assertion is true but the reason is false, which disagrees with known information about Venus`s atmosphere. Option 4 maintains that the statement about the improbability of human life on Venus is false, which contradicts current scientific understanding.