Assertion:
Benzonitrile is prepared by the reaction of chlorobenzene with potassium cyanide.
Reason:
Cyanide ${\text{C}}{{\text{N}}^ - }$ is a strong nucleophile.
A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
C. Assertion is correct but Reason is incorrect.
D. Assertion is incorrect but Reason is correct.

Hint: Haloarenes are prepared by halogenation of benzene. Halogenation of arenes is an electrophilic substitution reaction. The function of a halogen carrier is to generate electrophile which attacks the benzene ring to form haloarene.

Complete step by step answer:
Haloarenes are the compounds in which halogens are attached to the aryl compounds, i.e. aromatic hydrocarbons.
Chlorobenzene is an example for haloarene. Haloarenes are prepared by halogenation of benzene. Halogenation of arenes is an electrophilic substitution reaction. The function of a halogen carrier is to generate electrophile which attacks the benzene ring to form haloarene.
In haloarenes, the halogen gives a lone pair of electrons thereby producing a negative charge. Thus, it is delocalized. Since it has a negative charge delocalized around, so it is excess in negative charge. Therefore, it accepts only positive charge. When chlorobenzene is reacted with potassium cyanide, it is a nucleophilic substitution since cyanide is a nucleophile. There occurs electron-electron repulsion. Only electrophile can be attacked by haloarenes. Moreover, the carbon attached to the halogen is $s{p^2}$ hybridized. So, it is difficult to replace by nucleophile. Thus, the given assertion is not correct.
But the given reason is correct. Cyanide is a strong nucleophile.

Additional information:
Since reactivity is low, nucleophilic substitution reaction occurs at certain conditions, i.e. it can be replaced by hydroxyl ions. Nucleophilic substitution can be occurred if any electron withdrawing groups are substituted at ortho and para positions.


Note:
The presence of electron withdrawing groups at ortho and para positions withdraws electrons from the benzene ring. It helps to attack nucleophiles on haloarenes. Thus, carbon with positive charge is stabilized with resonance.