Assertion (A) : The boiling point of water decreases as the altitude increases. Reason (R) : The atmospheric pressure increases with altitude.

Assertion (A) is true: a liquid boils when its vapor pressure equals the surrounding atmospheric pressure, so reducing ambient pressure lowers the temperature at which vapor pressure equals that ambient pressure—hence boiling point falls with altitude [1]. The Reason (R) is false: atmospheric pressure does not increase with altitude; it decreases, often substantially (pressure drops with height at a known average rate), so the statement that pressure increases with altitude is incorrect [2]. Therefore R is not a correct explanation of A; the correct causal link is that atmospheric pressure decreases with altitude, causing the boiling point to decrease [2].

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 3: Geological Time Scale The Evolution of The Earths Surface > 3. Geological Time Scale – The Evolution of The Earth's Surface > p. 43
2. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 23: Pressure Systems and Wind System > Vertical Variation of Atmospheric Pressure > p. 305