Ascientific law

cannot be questioned
cannot be understood
cannot be discovered
cannot be broken

Cannot be broken. I think that's what it is.

A scientific law cannot be broken. This answer can be found simply through process of elimination. Anything can be questioned, which is how most laws are discovered, which brings me to my next point. Of course a scientific law can be discovered! How else would its existence be known? Finally, a scientific law can be understood as I understand (just as a basic example) Newton's First Law. This only leaves Choice D left.

Afine-grained igneous rock forms as a result of slow cooling.