Final answer:
Bob and Cathy use the separation strategy to balance stability with change by having a consistent birthday surprise with an unknown location each year. Option 2 is correct.
Step-by-step explanation:
Bob and Cathy are using the separation strategy for managing the stability-change dialectic in their relationship. By having a consistent event such as a birthday surprise each year, they adhere to a stable tradition. However, the change is introduced by not having Cathy know the location, which keeps the tradition fresh and exciting. This separation approach enables them to balance the need for stability through the yearly occurrence and change through an element of surprise.