Well defined nucleus is absent in
(a) blue green algae
(b) diatoms
(c) algae
(d) yeast

Blue-Green Algae is also known as cyanobacteria which is a photosynthetic bacteria. 

The cyanobacteria contain Chlorophyll-a which is a green pigment and other pigments such as carotenoids, and phycobilin. The blue-green bacteria have a nitrogen-fixing ability hence it has agricultural importance. It is used as a natural fertilizer in paddy fields for rice cultivation.