Match List - I with List II:
List-I         List-II
a. Li       i.   photoelectric cell
b. Na     ii.  absorbent of CO$$_2$$
c. K       iii. coolant in fast breeder nuclear reactor
d. Cs     iv.  treatment of cancer
              v.  bearings for motor engines
Choose the correct answer from the options given below:

We begin with the metal $$\text{Li}$$, lithium. Lithium is extraordinarily light and, when alloyed with other metals or when converted to lithium-based greases, it provides a low-friction, high-melting lubricant. Such greases are packed inside high-speed bearings of motor engines. Hence lithium corresponds to $$\text{bearings for motor engines}$$, i.e. entry $$v$$.

Now we consider $$\text{Na}$$, sodium. A very important property of liquid sodium is its exceptionally high thermal conductivity and its low neutron-absorption cross-section. Because of these two facts, liquid sodium is circulated through the core in many fast breeder nuclear reactors to remove heat. Therefore sodium must be matched with $$\text{coolant in fast breeder nuclear reactor}$$, i.e. entry $$iii$$.

Next comes $$\text{K}$$, potassium. Potassium hydroxide, $$\text{KOH}$$, readily reacts with carbon dioxide according to the equation $$\text{2 KOH + CO}_2 \longrightarrow \text{K}_2\text{CO}_3 + \text{H}_2\text{O}.$$ Because of this ready absorption, potassium compounds are widely used in air-purifying canisters and other devices as an $$\text{absorbent of CO}_2$$. So potassium matches with entry $$ii$$.

Finally, for $$\text{Cs}$$, caesium, we recall its very low ionisation enthalpy. When a caesium-coated surface is exposed to light, electrons are emitted even under weak illumination. This makes caesium ideal for constructing $$\text{photoelectric cells}$$, which convert light directly into electric current. Thus caesium corresponds to entry $$i$$.

Collecting all the matches we have obtained:

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

This sequence corresponds exactly to Option D in the given list.

Hence, the correct answer is Option D.