Somatic cells are defined as:

Somatic cells are indeed defined as those cells that form the body's organs and tissues. This definition encompasses a vast array of cells that make up the structural and functional components of an organism, excluding the germ cells, which are responsible for reproduction. In humans and many other organisms, somatic cells include everything from skin cells to muscle cells to nerve cells.

While reproductive cells (or germ cells) are crucial for biological reproduction and genetic continuity, they are not classified as somatic cells. The choice referencing all reproductive cells is therefore not accurate in this context. Protein synthesis involvement is a function that can be carried out by many types of cells, including somatic cells, but it's not a defining characteristic of somatic cells themselves. Immune cells, as a specific type of somatic cell, also do not encompass the broad category defined by somatic cells since they represent only one functional group within the larger body of somatic cells. Hence, the correct answer focuses precisely on the primary role of somatic cells in constituting the physical form of an organism.