A person using spectacles with a focal length of +1 m is unable to...

Using the lens formula:

$\dfrac{1}{v} - \dfrac{1}{u} = \dfrac{1}{f}$

Given the focal length of the spectacles $f = +1 \text{ m}$ and the maximum object distance $u = -1 \text{ m}$:

$\dfrac{1}{v} - \dfrac{1}{-1} = \dfrac{1}{1}$

$\dfrac{1}{v} + 1 = 1 \Rightarrow \dfrac{1}{v} = 0 \Rightarrow v = \infty$

The spectacles form the image of the object at $1 \text{ m}$ at infinity. Since the person can see this image clearly but nothing beyond it, their far point must be exactly at infinity.

A far point at infinity is the characteristic of an eye with normal vision.

For a nearsighted (myopic) eye, the far point is at a finite distance, so the maximum visible distance with these spectacles would be less than $1 \text{ m}$.

For a farsighted (hypermetropic) eye, the far point is virtual, allowing the person to see objects beyond $1 \text{ m}$ with these spectacles.

Therefore, this situation occurs if the person's vision is normal.