Answer : 3. "Pixels per inch"
Explanation :

1. PPI (pixels per inch) refers to display resolution, or, how many individual pixels are displayed in one inch of a digital image.

2. DPI (dots per inch) refers to printer resolution, or, the number of dots of ink on a printed image.

3. The physical size of an image refers to the dimensions in which it will print (ex: 8.5" x 11") or the pixel dimensions of an image displayed on the web (ex: 600 pixels x 800 pixels).