A group of cells performing similar function is called a _____.

The correct option is A Tissue
Animal cells and plant cells can form tissues, like muscle tissue. A group of similar cells performing similar functions is known as tissues. A tissue is an ensemble of similar cells and their extracellular matrix from the same origin that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues. So, the correct answer is 'Tissue'.