Which scenario represents a valid offer?

The scenario involving the poster on a lamppost offering £100 reward for a missing cat represents a valid offer because it clearly communicates a unilateral offer to the public. In this case, anyone who finds and returns the missing cat can accept the offer by performing the action of returning the cat, thereby creating a binding contract. This type of offer does not require any further negotiation; it is an open invitation to the public that can be accepted by anyone who fulfills the specified conditions.

Other scenarios, while they might suggest potential agreements, do not constitute valid offers in the legal sense. The new laptop advertised in the newspaper suggests an intention to sell but lacks the specificity required to establish a direct, immediate offer; it serves more as an invitation to treat. Similarly, the auctioneer's call for bids indicates that an invitation to negotiate is in place rather than an offer itself. Priced goods displayed in a shop window also represent an invitation to treat rather than a definitive offer, as the seller still holds the right to refuse to sell or negotiate the terms further.

Thus, the scenario of the reward poster stands out as it allows for clear acceptance through action by anyone who finds the missing cat, establishing a classic example of a unilateral contract.