My solution considers top of stack as end of array, thus more tests are needed

C#: method name should be PascalCase (Please refer to implementation of backward compatibility here )

Unnecesary System import ;-)

It should be stated that the top of the stack is at the beginning of the array, and not at the end.

It's not clear whether 2 sequences should be equal or it's allowed to use big plates for small meals too.

I like your general approach and structure for readability but I'm struggling to mark this as best practice, as the variable names are no good at all, but the way you iterate over the digits is pretty clever.

Fixed.

it won't run faster!

You can google "bouncy number" that is given in the description and find some difficult-to-understand mathematical description like this: http://www.mathblog.dk/project-euler-113-googol-not-bouncy/ and adapt it for the task given.

this code uses less abstactions which makes run much faster then most of the shorter metods I have seen.

List is a really strong function!

That's definitly too long for such a simple exercise.
Would be better to split the string by ' ' and then iterate through this and take the min.

In the C# Version the class name doesn't match with the test cases. The class name is CountCapitals and the test cases look for Captials.

[C# Code] I don't know how to use this fct in Oper:

In the test cases it just says "testing(Opstrings.Oper(Opstrings.HorMirror, "lVHt\nJVhv\nCSbg\nyeCt"), "yeCt\nCSbg\nJVhv\nlVHt");"

What parameter do I have to use for ... fct?

EDIT: I now just completed it. For you who have the same problem: just google "(language) function as parameter"

Any further information? Have you tried it again afterwards, maybe just a server issue?

Else try optimizing your code.