Final answer:
The correct value to set for the height property to ensure that the image remains proportional to its original size is 'auto'
Step-by-step explanation:
The correct value to set for the height property in order to ensure that the image remains proportional to its original size is auto.
When auto is set as the value for the height property,it allows the browser to automatically adjust the height of the image based on its original aspect ratio. This means that the image will scale proportionally with the specified width without distorting its original dimensions.