For the following Assertion and Reason, the correct option is:
Assertion (A): When Cu (II) and sulphide ions are mixed, they react together extremely quickly to give a solid.
Reason (R): The equilibrium constant of $$Cu^{2+}(aq) + S^{2-}(aq) \rightleftharpoons CuS(s)$$ is high because the solubility product is low.

We have to analyse both the given statements separately and then see whether the second one actually explains the first.

First we look at the chemical fact stated in the Assertion. When aqueous $$Cu^{2+}$$ ions meet aqueous $$S^{2-}$$ ions, the following precipitation reaction may occur:

$$Cu^{2+}(aq) + S^{2-}(aq) \rightleftharpoons CuS(s)$$

The solubility product $$K_{sp}$$ of $$CuS$$ is extremely small (of the order of $$10^{-36}$$). Because of this minute solubility, the moment both ions are present together in significant concentrations the ionic product $$[Cu^{2+}][S^{2-}]$$ very quickly exceeds $$K_{sp}$$ and the salt precipitates. Experimentally, a dark black solid of $$CuS$$ is indeed observed almost instantaneously. So the Assertion that “they react together extremely quickly to give a solid” is true.

Now we examine the Reason. For the same equilibrium we can write two ways of describing the system:

1. Dissolution direction (solid to ions):

$$CuS(s) \rightleftharpoons Cu^{2+}(aq) + S^{2-}(aq) \qquad K_{sp}$$

2. Precipitation direction (ions to solid):

$$Cu^{2+}(aq) + S^{2-}(aq) \rightleftharpoons CuS(s) \qquad K_{eq}$$

By definition we have the relation

$$K_{eq} = \frac{1}{K_{sp}}.$$

Because $$K_{sp}$$ is very small, its reciprocal $$K_{eq}$$ is correspondingly very large. Hence the Reason correctly states that “the equilibrium constant is high because the solubility product is low.” Therefore the Reason is also a true statement.

The crucial point is to decide whether this Reason actually tells us why the reaction is “extremely quick.” The magnitude of an equilibrium constant speaks about the thermodynamic feasibility (how far the reaction will go once equilibrium is reached), whereas the rate of the reaction depends on kinetic factors such as activation energy, collision frequency, diffusion of ions, etc. A very large $$K_{eq}$$ ensures that almost all $$Cu^{2+}$$ and $$S^{2-}$$ present finally become $$CuS$$, but it does not automatically guarantee that this conversion happens at an “extremely fast” speed. In aqueous solution ionic precipitation reactions are indeed rapid, but the fundamental reason for their speed is low activation energy and the fact that oppositely charged ions collide frequently, not the value of $$K_{eq}$$ itself.

So, while both statements are individually true, the Reason does not provide the correct explanation for the Assertion.

Hence, the correct answer is Option C.