The correct answer is option 1: Tissue. A group of similar cells that work together to perform a specific function is called tissue. Tissues are the building blocks of organs in the body.

Option 2: Organ, refers to a structure made up of different types of tissues that work together to perform a specific function. However, the question asks for the name of a group of similar cells, not an organ.

Option 3: Organ system, refers to a group of organs that work together to perform a specific function. This option is not the correct answer because it is a higher level of organization than tissues.

Option 4: Cellular organization, is not the correct answer because it is a general term that does not specifically refer to a group of similar cells performing a specific function.

In conclusion, option 1: Tissue is the correct answer as it accurately describes a group of similar cells performing a specific function.