What is the minimal distance between two points in histological specimen, which is distinguishable in a light microscope like two separate points which are not fused?

What is the minimal distance between two points in histological specimen, which is distinguishable in a light microscope like two separate points which are not fused?

1.  Ultramicroscopic unit


2.  Morphometric unit


3.  Artefact


4.  Angstrom unit


5.  *Microscope's resolution