The project needs to be completed by the end of the month, the deadline I believe is August 30th.
A.
The project needs to be completed by the end of the month. The deadline I believe is August 30th.
B.
The project needs to be completed by the end of the month. The deadline, I believe, is August 30th.
C.
The project needs to be completed by the end of the month; the deadline, I believe, is August 30th.
D.
Both B and C are correct.