An example of a weak electrolyte is:
A. ${\text{NaOH}}$
B. ${\text{Ba}}{\left( {{\text{OH}}} \right)_2}$
C. ${\text{HF}}$
D. ${\text{HI}}$

Hint: Electrolyte forms ions so, conduct electricity. The strong acid or base will form a strong electrolyte. Weak acid or base will form weak electrolytes. We will find the weakest acid or base from the given substances to determine the weak electrolyte.

Complete step by step solution:
The substance can be acids or bases which on dissolving in water dissociated forming ions and produce electricity are known as an electrolyte.
Electrolytes are of two types:
-Weak
-Strong
Weak electrolyte- The electrolytes that do not dissociate completely in an aqueous solution are known as a weak electrolyte. Weak electrolytes dissociate partially so, the solution of weak electrolytes contains ions as well as un-dissociated molecules. Weak acids or bases form weak electrolytes.
Strong electrolyte- The electrolytes that dissociate completely in an aqueous solution are known as a strong electrolyte. The solution of strong electrolyte contains ions only. Strong acids or bases form strong electrolytes.
Sodium hydroxide is a strong base. It dissociates completely in water and produces sodium and hydroxide ions. So, it is not a weak electrolyte.
The dissociation of sodium hydroxide is shown as follows:
${\text{NaOH}}\, \to \,{\text{N}}{{\text{a}}^{{\text{ + }}\,}} + \,{\text{O}}{{\text{H}}^ - }$
Barium hydroxide dissociates completely in water and produces barium and hydroxide ions. So, it is not a weak electrolyte.
The dissociation of barium hydroxide is shown as follows:
${\text{Ba}}{\left( {{\text{OH}}} \right)_2}\, \to \,{\text{B}}{{\text{a}}^{{\text{2 + }}\,}} + \,2\,{\text{O}}{{\text{H}}^ - }$
Hydrogen fluoride is a weak acid. It does not dissociate completely in water. So, it is a weak electrolyte.
Hydrogen iodide is a strong acid. It dissociates completely in water and produces hydrogen and iodide ions. So, it is not a weak electrolyte.
The dissociation of hydrogen iodide is shown as follows:
${\text{HI}}\, \to \,{{\text{H}}^{{\text{ + }}\,}} + \,{{\text{I}}^ - }$
So, hydrogen fluoride is a weak electrolyte.

Therefore, option (C) ${\text{HF}}$ is correct.

Note: The fluoride formed by dissociation of hydrogen fluoride is very small, so it cannot bear extra negative charge, so hydrogen fluoride is a weak acid. The acid or base which easily gives hydrogen or hydroxide ions are known as strong acid or base, so strong acid or base dissociates completely hence are known as a strong electrolyte. The acid or base which hardly gives hydrogen or hydroxide ions are known as weak acid or base, so weak acid or base does not dissociate completely hence are known as a weak electrolyte.