Explanation:

The correct answer is option 'B': An agreement to sell.

An agreement to sell refers to a contract where the seller agrees to transfer the ownership of goods at a later date, subject to certain conditions. In such a contract, the goods are unascertained, meaning they are not specifically identified or determined at the time of making the contract. The identification of the goods will occur at a later stage, typically before the actual transfer of ownership.

Let's break down the answer into headings and key points:

1. Definition of unascertained goods:
- Unascertained goods are goods that are not specifically identified or determined at the time of making the contract.
- The identification of the goods will happen at a later stage, usually before the transfer of ownership.

2. Definition of an agreement to sell:
- An agreement to sell is a contract where the seller agrees to transfer the ownership of goods to the buyer at a later date, subject to certain conditions.
- The transfer of ownership will occur once the goods are ascertained and the conditions specified in the contract are fulfilled.

3. Difference between sale and agreement to sell:
- In a sale, the ownership of the goods is immediately transferred from the seller to the buyer. The goods are ascertained and specifically identified at the time of making the contract.
- In an agreement to sell, the transfer of ownership is deferred until a later date, usually upon fulfillment of certain conditions. The goods are unascertained at the time of making the contract.

4. Contingent contract:
- A contingent contract is a contract that depends on the occurrence or non-occurrence of a specific event.
- While an agreement to sell may have some contingent elements, it is not primarily a contingent contract as it does not solely depend on the occurrence or non-occurrence of a specific event.

5. Wagering contract:
- A wagering contract is a contract where two parties agree to pay a sum of money or something of value based on the outcome of an uncertain event.
- An agreement to sell is not a wagering contract as it does not involve payment based on the outcome of an uncertain event.

In conclusion, when goods are unascertained, the correct contract is an agreement to sell. The transfer of ownership will happen at a later date once the goods are identified and the conditions specified in the contract are fulfilled.