The actual butterfly and the model are similar.
The ratio of corresponding parts of similar figures is the same.
The ratio of the fly's wingspan to that of the model is 8 to 32 or 8/32.
The ratio of the fly's body to that of the model is 3 is to x where x is the length of the fly model's body.
3/x = 8/32
That proportion does not appear in the options but a proportion that is equivalent to it does.
3/x = 8/32 cross multiplies to 3*32 = 8*x
Looking at option D which is 3/8 = x/32, notice that it cross multiplies to 8*x = 3*32 .
Therefor, option D is the same as the proportion and is the answer to the question.