Which sentence is written using passive voice?

a. the owner persuaded his staff to stay positive and to be careful not to make simple mistakes.
b. after making an error, the secretary could not forgive herself and she gave her two weeks’ notice.
c. the owner of the company reprimanded the secretary about her mistake but allowed her to keep the job.
d. the issue was caused by the secretary’s failure to file the company’s merger request on time.