Where should summarization occur to provide the best summarization and optimal paths during a single-failure incident as well as during normal operation?

Refer to the exhibit.

In this network design, where should summarization occur to provide the best summarization and optimal paths during a single-failure incident as well as during normal operation?
A. a single identical summary for all the branch offices placed on routers 1A, 1B, 2A, and 2B
B. two summaries on 1A and 1B, and two summaries on 2A and 2B
C. a single identical summary on 3A and 3B
D. a single summary on each aggregation device for the branches connected to them