3A) Hardness is the capacity to resist against scratch. A mineral can be very hard yet still be easy to BREAK, Whereas roughness or strength is resistance to breaking.
Taking the example of Diamond to understand it :-
Diamond is a hardest mineral known. It will scratch every other mineral. But diamond is not tough. It’s easy to shatter a diamond.
3B) Folds in a rock mass signifies the effort of the rock in a particular environment to adjust themselves to the changes taking place around them.Folds are created in rocks when they are under higher pressure and temperature.
3C) There are many ways through which rock strength is estimated, they are :-
1-Point load index
2- Unconfined compression test
3- Triaxial compression test
4- Brazilian test
5- Direct shear test