Hey there! :D
Organization in organisms is like this:
cells => tissues => organs => organ systems => organisms
Since we are talking about a group of cells, this would be a tissue. Don't let the word 'function' confuse you. Tissues do functions as well as an entire organ or organ system.
It is helpful to use the flow chart when asked questions like these, so make sure to memorize it! It will be helpful in any level of science.
I hope this helps!
~kaikers