If your DC got a D in maths GCSE then they will need to resit GCSE maths. If they got below a D, then they can take functional skills instead, but not all schools offer this.
The pass rate for 17 year olds resitting GCSE maths is a dismal 35.8%
The first chance to resit is in November, with results in January. If they fail in November, then the next chance is June. If they pass in November, that frees up the rest of the year to concentrate on other subjects. If not, it means maths lessons, maths homework and preparing for exams in June when they have other exams going on.
If they fail in June, then they need to continue resitting in Y13.
Students should try as hard as they can to pass in November. Unfortunately, I've taught a lot of resit classes and this is unusual. They forget a lot of maths over the summer. Then they spend time settling into their sixth form courses. Then November comes around really quickly and they suddenly start thinking about maths but it's too late.
If your DC needs to resit maths, I would suggest that they start preparing for November now, in order to counter the backwards slide over the summer.
There are some very useful websites here:
corbettmaths.com/5-a-day/
www.hegartymaths.com
www.mrbartonmaths.com/gcse.htm
I know a lot of students will see their other courses as their priority, but I know my resit class this year spent a lot of time wishing they'd got it out of the way in November.