Where both parties to an agreement are under a mistake as to a matter of fact essential to the agreement, it is:

Correct option is B

Under Section 20 of the Indian Contract Act, if both parties are mistaken about a fundamental fact, the agreement is void.
Information Booster:
Mutual mistakes regarding essential facts render the contract void, meaning it has no legal effect from the beginning.
Additional Information:
· Option (a): Voidable contracts can be enforced at the option of one party, which isn’t the case here.
· Option (c): Mistaken contracts aren’t illegal but are void.