Is anaebaena azollae gram positive or gram negative?

Anaebaena azollae is a cyanobacterium from Nostocaceae which grows in tempered and tropical regions. Used by humans in the agriculture.
It is phototrophic, it means it can absorb light to produce energy, and chemotrophic.
It cannot be colored by Gram coloration, it means it's neither gram-positive nor gram-negative bacteria. Under phosphorus starvation, It appears green on the microscope.
It should be noted that not all the bacteria can be classed in gram-positive and gram-negative bacteria. for example, Mycobacterium tuberculosis is also not colorable with Gram coloration, so we need to use different coloration technics to see it in the microscope.

The correct answer is d