The aspect ratio of an image is defined as
Answer : The ratio of width to its height measured in unit length
The aspect ratio of an image describes the proportional relationship between its width and its height. It is commonly expressed as two numbers separated by a colon, as in 16:9.
Reference : Aspect Ratio
Generally Aspect ratio is defined the ratio of width to its height ( or length)measured in unit length
so ans is A
P.S. However aspect ration depends upon the convention followed it can be ratio of height to width too