C. is correct.
A. She hasnt let me know if it's time to leave yet. - the hasnt should be hasn't
B. She hasn't let me know if it's time to leave yet. - this one has no errors
C. She hasn't let me know if its time to leave yet. - "it is" is spelled it's
D. She hasnt let me know if its time to leave yet. - the "its" is wrong and "hasnt"
