option 1
 line a
 line b
 line c
option 3
 line x
 line z
 line y
option 2
 a1
 a4
 a2
 a3

option 1
 line a
 line b
 line c
option 2
 a1
 a2
 a3
 a4
option 3
 line x
 line y
 line z

static IEnumerable<string> Sort(IEnumerable<string> unsorted)
    {
        string option = null;
        return
            from line in unsorted
            let isSubitem = line.StartsWith(" ")
            let parent = isSubitem ? option : option = line
            orderby parent, isSubitem, line
            select line;
    }

static IEnumerable<string> Sort(IEnumerable<string> unsorted)
    {
        string option = null;
        return
            unsorted
            .OrderBy(line => line.StartsWith(" ") ? option : option = line)
            .ThenBy(line => line.StartsWith(" ") ? line : null);
    }

yourEnumerable.OrderBy(y => y.Key).ThenBy(y => y.Prop2)

string lineRead;
string current;
var dictionary = new Dictionary<string, List<string>>);
while((lineRead = reader.ReadLine())
{
    if(!lineRead.StartsWith("  "))
    {
        dictionary.Add(lineRead, new List<string>());
        current = lineRead;
    }
    else
    {
        dictionary[current].Add(lineRead);
    }
}

dictionary.Keys.ToList().ForEach(y => dictionary[y] = dictionary[y].OrderBy(x => x).ToList());
dictionary.OrderBy(y => y.Key);

.OrderBy(x => x.option)
                    .ThenBy(x=>x.secondary)

list.OrderBy(<a delegate that will sort your array>).ThenBy(<a delegate that will sort your array again>)