There are a couple of ways to approach this question because of the unique form of Atwood's short story "Happy Endings." The first possible seed sentence is the set of lines that opens the story, before the choices (A, B, C, etc.) the reader selects for how the story ends.
The narrator begins this experimental text by writing,
John and Mary meet. What happens next?
This is the "seed" of the story's idea: what are the different possibilities for how the relationship between John and Mary will play out? The reader then explores a series of potential outcomes, with the narrator already telling us that "A" is the "happy ending" option. The seed sentence provides the base idea, and then the scenarios in each lettered section develop the idea with details.
Another way to approach this question is to suggest that each lettered scenario has its own seed sentence. It starts with a premise and then develops that premise with detail. For example, scenario B begins like this:
Mary falls in love with John but John doesn't fall in love with Mary.
The narrator then goes on to develop in detail the fallout from these two contradictory emotions. If you think of the seed sentence in this way, you could simply identify the seed at the start of each lettered section and follow how Atwood develops that idea throughout the section.