D. 1.25 m

In a pinhole camera, the relationship between image size and object size is given by:
\(\frac{\text{Image Size}}{\text{Object Size}} = \frac{v}{u}\)
where \( v \) is the camera length. Given \( v = 25 \, \text{cm} = 0.25 \, \text{m} \).

The image is one-fifth the object size, so the magnification is:
\(\text{Magnification} = \frac{1}{5} = \frac{v}{u}\)
Thus, we can rearrange this to find \( u \):
\(u = 5v = 5 \times 0.25 = 1.25 \, \text{m}\).