Match List I with List II:
List-I (Elements)   List-II (Properties)
(a) Li   (i) Poor water solubility of I$$^-$$ salt
(b) Na   (ii) Most abundant element in cell fluid
(c) K   (iii) Bicarbonate salt used in fire extinguisher
(d) Cs   (iv) Carbonate salt decomposes easily on heating
Choose the correct answer from the options given below:

We have four alkali-metal elements in List I: $$\text{Li},\; \text{Na},\; \text{K},\; \text{Cs}$$ and four characteristic properties in List II that must be matched one-to-one.

First, recall two fundamental periodic facts.

1. Down the group, hydration energy falls much faster than lattice energy.    Consequently the heavier alkali-metal halides (especially those with large anions such as $$I^-$$) become progressively less soluble in water.

2. Only lithium compounds often show behaviour resembling the alkaline-earth metals because the small $$\text{Li}^+$$ ion has an abnormally high polarising power.    Hence compounds such as $$\text{Li}_2\text{CO}_3$$ decompose readily on heating, a trait not shared by the carbonates of the heavier group members.

Now we examine each property one by one and attach it to the most appropriate element.

Property (iv) “Carbonate salt decomposes easily on heating.” The thermal decomposition reaction is $$\text{M}_2\text{CO}_3 \;\overset{\Delta}{\rightarrow}\; \text{M}_2\text{O}+ \text{CO}_2.$$ Among the alkali metals, only $$\text{Li}_2\text{CO}_3$$ executes this reaction at a moderate temperature; $$\text{Na}_2\text{CO}_3,\,\text{K}_2\text{CO}_3$$ and $$\text{Cs}_2\text{CO}_3$$ are heat-stable. So property (iv) must be allotted to $$\text{Li}.$$

Property (iii) “Bicarbonate salt used in fire extinguisher.” The well-known soda-acid and dry-powder extinguishers employ sodium bicarbonate: $$\text{NaHCO}_3 + \text{H}^+ \longrightarrow \text{Na}^+ + \text{H}_2\text{O} + \text{CO}_2 \uparrow.$$ Thus property (iii) belongs to $$\text{Na}.$$

Property (ii) “Most abundant element in cell fluid.” Physiology tells us that the intracellular fluid of animals and plants is rich in $$\text{K}^+$$ ions; sodium predominates outside the cell. Therefore property (ii) must match $$\text{K}.$$

Property (i) “Poor water solubility of $$I^-$$ salt.” Because hydration energy is smallest for the largest alkali-metal ion, $$\text{Cs}^+$$, its iodide $$\text{CsI}$$ is the least soluble of the series (while still appreciably soluble, it is markedly less so than $$\text{LiI}, \text{NaI}, \text{KI}$$). Hence property (i) is assigned to $$\text{Cs}.$$

Summarising the matches we obtained:

$$\begin{aligned} \text{Li} &\rightarrow (iv)\\ \text{Na} &\rightarrow (iii)\\ \text{K} &\rightarrow (ii)\\ \text{Cs} &\rightarrow (i) \end{aligned}$$

Looking at the given options, the sequence $$(a)-(iv),\;(b)-(iii),\;(c)-(ii),\;(d)-(i)$$ corresponds exactly to Option A.

Hence, the correct answer is Option A.