This virus, for the first time, was synthesized in the form of non-living crystals

The correct option is Tobacco mosaic virus.

Detailed Explanation:

Viruses were long considered mysterious entities because they occupy the borderline between living organisms and non-living matter. A key breakthrough in understanding their physical nature occurred in 1935 when the American biochemist and virologist Wendell Meredith Stanley successfully crystallized the Tobacco Mosaic Virus (TMV).

Stanley demonstrated that the virus could be obtained in the form of active, non-living crystalline proteins that retained their infectious properties when reintroduced to a host plant. This landmark discovery proved that viruses are composed of chemical macromolecules (specifically, nucleoproteins consisting of RNA wrapped in a protein coat) rather than being typical cellular organisms. For this pioneering work, Wendell Stanley was awarded the Nobel Prize in Chemistry in 1946.

Let's look at why the other options are incorrect in this context:
1. Pox virus: Although it is a large and complex animal virus, it was not the first to be crystallized.
2. Flu virus: The Influenza virus is an enveloped virus that was isolated and characterized later, but it was not the subject of the first crystallization experiments.
3. Bacteriophage: These are viruses that infect bacteria. While they have been studied extensively to understand molecular biology and genetics, they were not the first to be crystallized in non-living form.