In a relational database, a 'Primary Key' in one table is used to uniquely identify each record in that table. To relate this to another table, this key is used in the second table as a 'Foreign Key'. Therefore, the 'PatientID' from the 'Patients' table should be included in the 'Appointments' table as a 'Foreign Key' to establish a relationship between patients and their appointments. Option A is correct because it maintains referential integrity by ensuring that every appointment record is associated with a valid patient record. The other options do not correctly establish this connection within the context of a relational database structure.