a) Between pH 1 and 6. This is because within this range, the enzyme activity is nonzero, whereas outside the range it is zero
b) Optimum pH. This is simply because it is the pH for which an optimum enzyme activity is achieved, maximising the rate of reaction
The final question requires a little more explanation. At this level we work with the lock and key hypothesis, that is to say the enzyme only catalyses the reaction when the substrate(s) collide with the active site with the correct orientation and minimum activation energy is exceeded. The enzyme is specialised to the substrate, so the active site (where catalysis by adsorption/desorption occurs) is a very specific shape to fit it. In denaturing, the active site changes shape such that it is no longer specialised to the substrate. The lock is no longer the right shape for the key, so the enzyme no longer works.
This denaturing can be caused by extremes in pH, where ionic interactions with H+ or OH- break bonds in the protein, or by high temperature breaking these bonds.
I hope this helps you :)