Final answer:
Groups of similar cells that perform related tasks in multicellular organisms are known as tissues, which further combine into organs and organ systems, culminating in the complete organism.
Step-by-step explanation:
In multicellular organisms, cells are organized into tissues, which are groups of similar cells that perform the same function. Tissues are further organized into organs, which are collections of tissues that work together to perform a specific function within the body. Multiple organs then combine to form an organ system, which is a higher level of organization where functionally related organs work together. Finally, all the organ systems within a multicellular organism come together to form the complete living entity, or organism.
Therefore, groups of cells performing related tasks are known as tissues, and the correct answer to the student's question is A) tissues.