Assertion: The amphoteric behaviour of water is explained by Lewis acid base theory
Reason: water acts as acid with $$NH_3$$ and base with $$H_2S$$

Analyzing the Assertion:

"The amphoteric behaviour of water is explained by Lewis acid base theory"

The amphoteric nature of water means it can act as both an acid and a base. This behaviour is best explained by the Bronsted-Lowry theory, not the Lewis acid-base theory. According to Bronsted-Lowry theory, water can donate a proton (acting as acid) or accept a proton (acting as base). Lewis theory defines acids as electron pair acceptors and bases as electron pair donors, which is not the primary framework used to explain the amphoteric behaviour of water.

Therefore, the Assertion is false.

Analyzing the Reason:

"Water acts as acid with $$NH_3$$ and base with $$H_2S$$"

With $$NH_3$$: $$H_2O + NH_3 \rightleftharpoons NH_4^+ + OH^-$$. Here, water donates a proton to $$NH_3$$, acting as an acid. This is correct.

With $$H_2S$$: $$H_2O + H_2S \rightleftharpoons H_3O^+ + HS^-$$. Here, water accepts a proton from $$H_2S$$, acting as a base. This is correct.

Therefore, the Reason is true.

Hence, the correct answer is Option D: Assertion is false but Reason is true.