Final answer:
The property/value pair that will ensure the background image of the div scales proportionally while filling the entire div is background-size: cover;
Step-by-step explanation:
The property/value pair that will ensure the background image of the div scales proportionally while filling the entire div is background-size: cover;.
This property value scales the background image to fill the entire div, while also maintaining the aspect ratio. It ensures that the image covers the entire background area, without any gaps or stretching.
For example, if you have a square div and a rectangular background image, the image will be cropped to fit the square div while maintaining its proportions.