Guys I have some questions please help me.,
1. if I make a tetris game out of c# will it be difficult to do?
2. Is it possible to finish it in less than a week ?
3. Also what should I use as a game studio? OpenGL, XNA or can you suggest any that is easier to use?
4. can you suggest any other ideas on how I can make the classic tetris game a better one? Like additional features or maybe some effects and how to put it on the game, can you help me make the game better? help me guyss

1 - Doesn't have to be.
2 - Possibly, depending on how much time it takes to make the graphics look good
3 - OpenGL has a steep learning curve, XNA is OK but for something simple like tetris simple GDI+ calls would probably be easiest
4 - There is no way to make tetris better, unfortunately. It is perfect in every way.

commented: is GDI+ the ones you just code in the visual C# itself? I mean you don't need to install anotther software just like the XNA? +2

All of that depends on your level of experience.
You're just going to need to pick something and do it.
Console games can be easier to write, if you're not familiar with graphics rendering and animation.

Given only one week, you need to pick something you're comfortable with or that you're willing to get done no matter what.