Assertion (A): Existence of human life on Venus is highly improbable.
Reason (R): Venus has an extremely high level of carbon dioxide in its atmosphere.

The correct answer is option A, where both the Assertion and Reason are correct and the Reason is the correct explanation for the Assertion.

Explanation:

Assertion (A): Existence of human life on Venus is highly improbable.
Reason (R): Venus has an extremely high level of carbon dioxide in its atmosphere.

To understand why option A is the correct answer, let's examine the assertion and reason individually.

Assertion (A): Existence of human life on Venus is highly improbable.
Venus, the second planet from the Sun, is known for its extreme conditions that make it inhospitable for human life. Some of the key factors that contribute to the improbability of human life on Venus are:

1. Extreme temperature: Venus has a surface temperature that averages around 900 degrees Fahrenheit (475 degrees Celsius), which is hotter than the surface of Mercury, even though Mercury is closer to the Sun. Such high temperatures would be detrimental to human life as it would be impossible to survive without protection.

2. Atmospheric pressure: Venus has an extremely dense atmosphere with a pressure about 92 times greater than Earth's atmospheric pressure at sea level. This high pressure can crush any structure and would be highly damaging to human bodies.

3. Lack of oxygen: Venus has a predominantly carbon dioxide atmosphere, with only trace amounts of oxygen. Oxygen is vital for human respiration, and the absence of sufficient oxygen would make it impossible for humans to survive.

4. Acidic atmosphere: Venus has a thick atmosphere comprised mainly of carbon dioxide, with clouds of sulfuric acid. The presence of sulfuric acid in the atmosphere makes the environment highly corrosive and unsuitable for human life.

Considering these extreme conditions, it is highly improbable for human life to exist on Venus without extensive technological advancements.

Reason (R): Venus has an extremely high level of carbon dioxide in its atmosphere.
The reason provided in the question is that Venus has an extremely high level of carbon dioxide in its atmosphere. This is indeed true, as Venus has a carbon dioxide-dominated atmosphere with a concentration about 96% by volume. The high levels of carbon dioxide contribute to the greenhouse effect on Venus, trapping heat and leading to the planet's scorching temperatures.

The presence of such high levels of carbon dioxide in the atmosphere further reinforces the assertion that human life on Venus is highly improbable. The excessive heat, along with the lack of oxygen and the corrosive nature of the atmosphere, makes it inhospitable for humans.

Therefore, option A is the correct answer as both the assertion and reason are correct, and the reason correctly explains why the existence of human life on Venus is highly improbable.