Answer
The object distance is between $f$ and $2f$
Work Step by Step
We can write the lens equation:
$\frac{1}{f} = \frac{1}{d_o}+\frac{1}{d_i}$
For a converging lens, when the object is located at $2f$, then the image is inverted and the image has the same size as the object.
If the object moves from $2f$ toward $f$, then the inverted image becomes larger than the object.
Since the image is inverted and larger than the object, the object distance is between $f$ and $2f$
