For some reason in the C# version, the third argument is a List but using System.Collections.Generic; is not there by default

In C# tests, test named "ExBad5" has a useless trailing comma in the last grid row.
Sorry, but I had to point it out :)

Why is the tested input of type object and not int in C# ?

The description says "Note: Input will either be a positive integer (or a string for untyped languages)."
I feel like it's unnecessary for an 8 kyu to do parsing. C and C++ versions are int for example, seems fine.

Sorry, after reading my post it sure doesn't make any sense.
What I meant is that Linq in .NET only handles jagged arrays natively, and not multidimensional ones like here
You have to use extra tricks to use it (see https://stackoverflow.com/questions/18896150/c-sharp-linq-return-a-multidimensional-array-from-linq and the best answer's source)

This is why I'd argue that using jagged arrays as returned values would be better, as it's handled by Linq by default
What do you think ? Linq is why peopple use C# here in the first place. It's a bit of nitpicking though, I'll admit

C# uses a rectangular array (int[n,x]) for some reason, I don't think this makes sense since the rows and columns are the same length.

Shouldn't it be a simple jagged array (int[n][]) ? I don't see any reason to use a rectangular one here.
It would also make Linq more accessible

First sentence in the description: Should have a space between "a" and "to"