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

Assertion (A) is true: human life on Venus is highly improbable because the surface environment is extremely hostile — a dense, mostly carbon dioxide atmosphere with clouds of sulfuric acid, surface pressures ~92 times Earth’s and searing temperatures — conditions incompatible with human physiology and surface missions [1]. Reason (R) is also true: Venus’s atmosphere is dominated by CO2, which drives an intense greenhouse effect. This runaway greenhouse trapped heat, boiled away oceans, and produced the present high temperatures and thick CO2 atmosphere and pressure that make the surface uninhabitable for humans [2]. Thus the high level of CO2 is the primary physical cause explaining why human life on Venus is highly improbable, so R correctly explains A.

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 2: The Solar System > Venus > p. 28
2. [2] https://ntrs.nasa.gov/api/citations/20030067857/downloads/20030067857.pdf