The solubility of AgCl will be maximum in which of the following?

A student needs to prepare a buffer solution of propanoic acid and its sodium salt with pH 4. The ratio of $${{[C{H_3}C{H_2}CO{O^ - }]} \over {[C{H_3}C{H_2}COOH]}}$$ required to make buffer is ___________.

Given : $${K_a}(C{H_3}C{H_2}COOH) = 1.3 \times {10^{ - 5}}$$

The K_sp for bismuth sulphide (Bi₂S₃) is 1.08 $$\times$$ 10^$$-$$73. The solubility of Bi₂S₃ in mol L^$$-$$1 at 298 K is :

Given below are two statements one is labelled as Assertion A and the other is labelled as Reason R :

Assertion A : The amphoteric nature of water is explained by using Lewis acid/base concept.

Reason R : Water acts as an acid with NH₃ and as a base with H₂S.

In the light of the above statements choose the correct answer from the options given below :