Which sentence is written correctly?

A. His mother offered to drive us to school, so his mother said we would have to walk home.
B. His mother offered to drive us to school, so, his mother said we would have to walk home.
C. His mother offered to drive us to school, but his mother said we would have to walk home.
D. His mother offered to drive us to school, and his mother said we would have to walk home.