I am a senior from Walnut High School, California. I got a D+ in my first semester of economics in my senior year, this is a single semester class, do I have to repeat this course. School counsellor said that I don’t have to but I am still anxious. Will it affect my college admission?
Assuming that you applied to UCs, you need to inform them:
http://admission.universityofcalifornia.edu/how-to-apply/after-you-apply/
You are likely to need to do that to other colleges that you applied to, unless they will see it anyway though a mid year report.
In April, you need to check the college you intend to matriculate to in order to see if the D+ in senior year will cause your admission offer to be rescinded.
Rule of thumb for many schools, is no D’s or F’s Senior year or you could jeopardize your college acceptances as stated by @ucbalumnus. If possible, I would repeat the class since it is only a Semester course and this is your final grade (no 2nd semester to show improvement).