Answer: Cells.
Explanation:
A tissue is a group of cells that have a similar structure and work together to have the same function. These groups of cells make up organs such as lungs, heart, skin or brain. So it is an organisational level between cells and a complete organ because it ensembles similar cells and their extracellular matrix. To fully understand this, we have to consider that a cell is the smallest structural and functional unit of all organisms. So when they are aggregated together, they form tissues.
