Question

When specialized cells are grouped together to form a common function, what they are called?

Answer 1

The correct answer to your question would be - D; Tissue.

When you have a group of specialized cells (which means they're performing a specific function and are not stem cells which have yet to differentiate to a certain specific role) that start grouping together and form a common function, the name you would give to such a group of cells would be a tissue. 

One example of a tissue could be a piece of a liver that you could cut off, or a piece of any organ. That piece is a homogenous group of specialized cells which all play an important and similarly specialized role in the functioning of that organ. 

Answer 2

The answer to this question would be the last one, tissue.

These specialized cells group together to form a common function which then becomes a tissue. These tissues then group together and form an organ. Organs then form together a system and systems form together an organism. These organisms gather and form a community and eventually into an ecosystem. This is typically a process as to how cells become organisms and how organism make up the ecosystem. Cells are the basic unit of life. Without them, there would be nothing at all.