Here's why:

* Altering genes in bacteria (specifically, the development of recombinant DNA technology) led to the birth of genetic engineering in the 1970s. This opened up entirely new possibilities for manipulating genes, leading to advancements in medicine, agriculture, and other fields.

Let's look at the other options:

* Light microscope: While essential in biology, the light microscope was developed long before the 1970s and didn't lead to a new branch of science in that period.

* Centrifuge: A centrifuge is a tool used to separate components based on density, and while important in various scientific fields, it wasn't a revolutionary technology in the 1970s that sparked a new branch of science.

* Chromatography: Chromatography is a separation technique used to isolate and identify components of a mixture. While useful, it didn't lead to a new branch of science in the 1970s.

Therefore, the technology that opened up a new branch of science in the 1970s was the ability to alter genes in bacteria, which gave rise to genetic engineering.