C

Explanation:

An offer is a definite proposal made with the intention to be bound by it upon acceptance. Option C, an advertisement offering a reward for finding a lost dog, is a classic example of a unilateral offer, as it promises a reward in exchange for a specific act (finding the dog), as established in cases like Carlill v. Carbolic Smoke Ball Co.. Option A (declaration of intention) is incorrect, as it lacks the intent to be bound. Option B (invitation to offer) is incorrect, as it invites others to make offers (e.g., advertisements for sale). Option D (offer made in a joke) is incorrect, as it lacks serious intent. The answer “C” aligns with contract law principles.