Why the correct answer is right
In a finishing lab, "blocking" is the process of attaching a disposable holder, or "block," to the lens so that it can be mounted in the edger. A machine called a blocker is used to do this. The technician places the lens in the blocker, aligns its optical center and axis to the correct position, and then the machine adheres the block to the lens front surface with an adhesive pad. This block is what the edger's chuck grips to hold and spin the lens during the cutting process.
Why the other options are incorrect
Coatings, power measurement, and frame tracing are all separate processes in the fabrication of a pair of glasses.
Memory aid
You **block** the lens so the edger doesn't **balk** (i.e., so it has something to hold on to).
Real-world application
The blocking step is where the optician's measurements (PD and OC height) are physically applied to the lens. The accuracy of the entire finished product depends on the precision of this step. An error in blocking will result in a lens with a misplaced optical center, which will fail final inspection and have to be remade.
