Hi Janet
Yes, I agree with the others. In the old days of O' levels, A-C was a pass and anything else was a fail. When GCSEs were introduced it seems that A-C grades were the equivalent of the O' level A-C etc but schools and exam boards (presumably at the behest of the Government) do not class D etc as a fail. I think it was because it was felt that all pupils should have something to show for their time at school. I think that the only time that a D grade is acceptable is if you want to apply for some vocational college courses, such as animal care and beauty therapy.
I have a feeling though that if you are entered for Foundation Tier, it is possible to get a C. It does seem rather pointless entering someone for an exam where the max they can get is a D. I would check on that if I were you.