The correct answer is option 'A': Lien.

Explanation:
When a seller has not received payment for the goods and the goods have not been delivered to the buyer or his agent, the unpaid seller has a right of lien. Let's understand what this means:

Lien:
A lien is a legal right that allows a person to retain possession of someone else's property until a debt owed by that person is paid off. In the context of an unpaid seller, a lien allows the seller to retain possession of the goods until the buyer makes payment for them.

The right of lien is a powerful tool for the seller to secure payment for the goods. By retaining possession of the goods, the seller effectively prevents the buyer from using or selling the goods until the payment is made.

The right of lien can be exercised in two situations:

1. Goods in possession of the seller:
If the seller has physical possession of the goods, the seller can retain possession until payment is made. This means that the seller can refuse to deliver the goods until the buyer pays the outstanding amount.

2. Goods in transit:
If the goods are in transit, meaning they have been dispatched by the seller but have not yet reached the buyer, the seller can exercise the right of stoppage in transit. This allows the seller to stop the goods in transit and regain possession of them until payment is made.

It is important to note that the right of lien is only available to the unpaid seller. Once payment is made, the seller loses the right to retain possession of the goods and must deliver them to the buyer.

In conclusion, the correct answer is option 'A': Lien. This right allows the unpaid seller to retain possession of the goods until payment is made by the buyer.