Step 1: Think about what magnification is meant to tell us.
Magnification is simply a number that tells us how the size of the image compares with the size of the actual object.
Step 2: Try a quick example.
Suppose an object of height $2\text{ cm}$ forms an image of height $3\text{ cm}$. The magnification here is $\dfrac{3}{2} = 1.5$, which we get by dividing the image height by the object height, not the other way round.
Step 3: Write it as a general formula. \[ m = \frac{h_i}{h_o} \] where $h_i$ is the height of the image and $h_o$ is the height of the object.
Step 4: Match this with the given options.
This is precisely the ratio of the height of the image to the height of the object.
\[ \boxed{\text{ratio of height of image to height of object}} \]