Muscle tissue is primarily made up of cells. Muscle cells, also known as myocytes, are specialized cells that can contract and generate force. These muscle cells are organized into muscle fibers and can be found in three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle.
While muscle tissue is a component of organs (such as muscles themselves), it is fundamentally composed of individual cells. So, among the options provided, the correct answer is cells.