These are ‘multicellular grade’ entity

The correct option is Sponges.

To understand why this is the correct answer, let's break down the levels of organization in multicellular organisms and analyze the options provided:

1. Levels of Biological Organization:
In the animal kingdom, multicellular organisms exhibit different structural levels of organization depending on how their cells are grouped and specialized:
• Cellular level of organization (Multicellular grade): The body consists of a loose aggregation of cells. These cells exhibit division of labor, but they do not form distinct tissues or organs. This is the lowest grade of multicellular organization.
• Tissue level of organization: Cells performing the same function are arranged into tissues (e.g., Coelenterates).
• Organ/System level of organization: Tissues are grouped together to form organs and organ systems (e.g., Vertebrates).

2. Analyzing the Options:
• Prokaryotes: These are unicellular organisms (like bacteria) lacking a defined nucleus and membrane-bound organelles. They do not belong to the multicellular category.
• Coelenterates (Cnidarians): These organisms have reached the tissue level of organization, which is a higher grade than the basic cellular multicellular grade.
• Vertebrates: These are highly complex animals with well-developed organ systems (organ-system level of organization).
• Sponges (Phylum Porifera): Sponges are multicellular animals, but their cells are loosely arranged and do not form tissues. Therefore, they exhibit the cellular level of organization, making them the classic example of the basic "multicellular grade" of construction.