Answer:
The answer is "Halftone".
Step-by-step explanation:
The technological breakthrough mentioned in the question is the halftone printing method used for the printing of photographs. The technique took advantage of dotting when printing the photos. Instead of using continuous lines, they used dots that differed either in size or tones of black which from a distance, was not observable and looked like a regular photograph but it helped cut down the amount of ink used greatly.
I hope this answer helps.