I would go with option A:
String.Join(String.Empty, los.ToArray());
My reasoning is because the Join method was written for that purpose. In fact if you look at Reflector, you’ll see that unsafe code was used to really optimize it. The other two also WORK, but I think the Join function was written for this purpose, and I would guess, the most efficient. I could be wrong though…
As per @Nuri YILMAZ without .ToArray(), but this is .NET 4+:
String.Join(String.Empty, los);