Look... C is one of the prominent programming languages. I won't say that u will learn C completely through Sololearn, but u need to start at some point using some material. I'm also learning C from here and it's a great app to develop coding skills in u. Don't be too fast. Do each and every topic perfectly and explore more in it. This is the only thing which I can suggest u. It fully depends on u and your determination of learning C. Be confident on yourself and also on this app. All the best!
You can learn a lot about c coding using sololearn. And when you'll have completed sololearn c tutorial, that also doesn't means you've learned complete c. If you wanna do expertise yourself in c coding, you have to learn and practice with other sources too!!!