This is the structural and functional unit of all living organisms, and are sometimes called the "building blocks of life."

The structural and functional unit of all living organisms are cells.  

Explanation:

Cells are the smallest level of organism such as: humans, animals, plants fungus among many others. At this level of living organisms it is possible to achieve process to keep the organism alive such as: metabolic process, respiration, and so on. Due to it, cells are called the "building blocks of life"

Let's consider an animal, plant or human example:  

If you take several cells and join them together, they can form different types of tissues. Those form the building blocks for plant structure or animal organs.  

When you join several tissues, you can form an organ. For example: Hearth. If  we put together several organs, they can form a whole system. For example: circulatory system.

When you put several systems together, you can get the whole living organism: Circulatory system, digestive system, among others form the human body.

Besides of the above, cells can achieve respiration process, metabolic process among many others. Due to it, they are the structural unit of all loving organisms.  

i think that the best answer would be c.

tell me if im wrong : )