Which layer of the hierarchical network design model is refered to as the high-speed backbone of the internetwork, where high availability and redundancy are critical?

access layer
core layer
data-link layer
distribution layer
network layer
physical layer

ccna exploration 3 chapter 1 Exam Question And Answers 

Which layer of the hierarchical network design model is refered to as the high-speed backbone of the internetwork, where high availability and redundancy are critical?