C and D is almost the same as A and B. A little bit harder of course, but they are still algorithmic problems.
What kind of advice do you want to hear? 'Do that and this and you will solve almost every C and D problem'? I think that nobody can give you such an advice.
Keep training.

learn basic algorithms and sort problems by number of solvers....

I think this is better because there are some B problems harder than some C problems most of A problems can be solved without studying algorithms but most of B,C and D problems need algorithmic knowledge and data structures.

learn these topics:

• dynamic programming

• graph algorithms

• data structures :set,vector,queue,....

most of C and D problems can be solved simply if you know these topics especially data structures.....

and finally practice,practice and practice....

sorry for my bad English.

It's more useful to solve problems, than creating this kind of blogs