Jane missed several assignments, she knew her grade would be lowered, but she didn't expect a D.
A.
Jane missed several assignments; although she knew her grade would be lowered, she didn't expect a D.
B.
Jane missed several assignments, she knew her grade would be lowered but she didn't expect a D.
C.
Jane missed several assignments. She knew her grade would be lowered, but she didn't expect a
D.
Both A and C are correct.