Hello and thanks for using Just Answer.
I am Al and am happy to assist you with your enquiry.
Class C (i) land charge is what is called a puisne mortgage which is a legal mortgage which is not secured by a deposit of documents relating to the legal estate affected.
Therefore, your parents must have taken out a second mortgage or loan at some stage, and as it is still showing in the Land Charges department, you need to ascertain who this charge was with, so they can remove it (on the basis that it has been repaid).
I hope this assists and answers your question.