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
Picture a suction-cup handle stuck onto a large glass panel so a worker can grip it and feed it into a cutting machine. Without that handle, the glass would spin free and the cut would be ruined. In lens fabrication, the block is that handle — a small puck glued to the lens front so the edger's chuck has something to grab and hold steady during cutting.
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.
