Answer: The correct option is -
a. Animalia.
Step-by-step explanation:
Animalia is a kingdom of multicellular, eukaryotic organisms that lack a cell wall around cell membrane.
They have true nucleus and membrane bound subcellular organelles like mitochondria, endoplasmic reticulum, golgi complex, etc.
Example- Human beings, Dogs, Cats, Rats.
As per the given information in the question, cell under the microscope observed as attached to many other cells, and it has a nucleus and membrane bound organelles. it has no cell wall.
Thus, the organism of this cell is grouped into Animalia kingdom.