Besides paddy field, cyanobacteria are also found inside vegetative part of
A) Psilotum
B) Pinus
C) Cycas
D) Equisetum

Hint: Cyanobacteria are photosynthetic and aquatic, i.e., they live in the water and can grow their own food. They are very small and typically unicellular since they are bacteria, but they sometimes develop in colonies large enough to see.

Complete answer:
The origin of plants is the other great contribution of cyanobacteria. Currently, the chloroplast in which plants make food for themselves is a cyanobacterium living within the cells of the plant.
Cyanobacteria started to take up residence within some eukaryotic cells sometime in the late Proterozoic, or in the early Cambrian, making food in exchange for a home for the eukaryotic host. This phenomenon is known as endosymbiosis, and the eukaryotic mitochondria is also the cause of it.
Cyanobacteria are also called 'blue-green algae' because they are photosynthetic and aquatic. This name is useful for speaking of organisms that make their own food in the water, but does not indicate any relationship between cyanobacteria and other algae called organisms.
Cyanobacteria are linked to bacteria, not eukaryotes, and only chloroplasts in eukaryotic algae are associated with cyanobacteria.
Few members present in the coralloid roots' algal zone are: Anabaena cycadae, Nostoc punctiforme, Oscillatoria, Pseudomonas radicicola, Azotobacter and even a few fungi. Blue green algae or Cyanobacteria (Anabaena, Nostoc and Calothrix) may rarely be present intracellularly (i.e. within the cell) in Cycas' coralloid roots, according to Kubitzki (1990).
He was of the opinion that these algae fix nitrogen and encourage host plant growth.The role assigned to the coralloid roots is primarily the fixation of nitrogen due to the presence of blue-green algal members and some nitrogen-fixing bacteria.

The correct answer is option(C) Cycas.

Note:They have the distinction of being the oldest fossils known, actually more than 3.5 billion years old! You would be shocked to know that cyanobacteria are still around; they are one of the largest and most significant bacterial groups on earth.