Answer:
Stipulation.
Step-by-step explanation:
In this scenario, Rex Garner recently made an offer to Harry Barns for the sale of his shop using a registered letter. The offer says that Harry "may accept by registered letter." This detail is an example of a stipulation.
A stipulation in business can be defined as a formal legal acknowledgment and agreement made between two or more groups of people (parties) before entering into a contract or business deal.
This ultimately implies that, a stipulation is a condition or clause used to convey agreement in a contract between two or more groups of people. The statement "may accept by registered letter." in the offer made by Rex Garner to Harry is a stipulation, conveying the message that Harry can only show agreement by using a registered letter as well.