I had to go to my Snell for this one (ps I hate you for making me open this book again). Alright, intraperitoneal means that the structure in question is completely surrounded by visceral peritoneum.
The rectum, ascending/descending colon are retroperitoneal. The superior rectal artery supplies the proximal rectum, so rule that out. The inferior mesenteric artery supplies the descending colon, so that can be ruled out too. It doesn't make sense to have the sigmoid colon be intraperitoneal when the two things it's connected to (dec. colon and rectum) are both retroperitoneal. So I think its safe to say you can rule that out as well.
That leaves us with the ascending left colic artery, which is a branch of the inferior mesenteric artery. Although I just said the inferior mesenteric artery is retroperitoneal, the ascending left colic artery anastomoses with the middle colic artery... a structure that I know for sure is intraperitoneal. The middle colic is a branch of the s. mesenteric artery (an intraperitoneal structure), and it supplies the transverse colon (another intraperitoneal structure) So I guess you can say the left colic is intraperitoneal by guilt of association with the middle colic.
Middle colic (intraperitoneal) <=anastomose=> ascending left colic
