Which level of organization is characterized by a group of cells that work together to perform a common function? i need help with this question

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!

the oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left heart, completing the pulmonary cycle. this blood then enters the left atrium, which pumps it through the mitral valve into the left ventricle. from the left ventricle, the blood passes through the aortic valve to the aorta.

deoxygenated blood leaves through the right ventricle through the pulmonary artery. from the right atrium, the blood is pumped through the tricuspid valve (or right atrioventricular valve), into the right ventricle.