You need Enumerable.SelectMany
Projects each element of a sequence to an IEnumerable and flattens the resulting sequences into one sequence
List<string> distinctList = listOfLists.SelectMany(r => r).Distinct().ToList();
You can also modify your code for adding elements in list like:
listOfLists.Add(new List<string> {"a", "b", "c", "c"});
instead of creating an array and then converting it to list.