Skeletal muscle cells are multinucleated from the fusion of muscle cells. Smooth muscle cells are strictly mononucleated, and cardiac muscle cells are mononucleated in humans. However, cardiac muscle cells are functionally syncytium. They have their cells interconnected with gap junctions, which allow rapid co-ordinated contraction of the muscle cells. The cells, however, are not multinucleated and are not true syncytium (as seen in case of skeletal muscle) but functionally they are syncytium (multinucleated). Therefore, cardiac muscle is functionally multinucleated and skeletal (voluntary) muscle are true multinucleated. So, the correct option is 'Cardiac muscle'.