Do you agree that "A cell is a building unit of an organism"?

Yes. Cell is a building unit of every living organism as every living being is made up of one or more cells. In unicellular or acellular organisms, the single cell performs all the functions of life. In multicellular organisms all the cells have a similar basic structure and perform similar basic life activities. However, they become specialised to form components of different structures that perform different functions. Cells are first organised into tissues, each with a specific function, e.g., contraction by muscular tissue. Tissues are organised to form organs with each organ performing a specific fonction, e.g., heart, stomach, kidney. Organs are grouped into organ systems, each with a major fonction, e.g.. circulatory system, excretory system, respiratory system. A living being has a number of organ systems. However, in all such organisational complexity, cell remains the basic building unit of the organism.