I have learned C programming several years ago, but I would like to refresh my knowledge so looking for some material, which is perfect for this purpose. I have found ALISON courses and I am looking for similar solutions like this, where I can also take tests.

The C Language as in what is in the K&R book isn't too hard to read and practice what's there.

It's when you go beyond what's in that book at move to real apps today that frankly I'm not using C except for embedded work and some legacy apps I wrote in 2000 to present day. All new stuff is not "C."

Tutorials and such are rather plentiful but I fear outside of embedded work or maybe drivers you may want to head to learn other languages.

Then again, I find folk that can write code but can't design an app.

You can always try udemy. But, be forewarned, you have to pay before you can watch their videos, However; on the upside, the site does have promise if you stick with it..

Or you can try lynda, which is another good site. Sadly, it is also a paid site.

Get a copy of K&R (Kernighan and Ritchie) and read a lot of code. Then, find a mentor you can talk to when you are having problems.