Trey is looking at a cell under the microscope. it is attached to many other cells, and it has a nucleus and membrane bound organelles. it has no cell wall. in which kingdom should this organism be classified? a) animalia b) archaebacteria c) fungi d) plantae

The cell at which Trey was looking at, belonged to an organism of Kingdom Animalia.
It could not be any other option because Plants have cell walls (made up of cellulose, hemicellulose, pectin etc.) , fungi also have cell wall (made up of a chemical called peptidoglycan).
Archaebacteria do not have membrane bounded organelles, cause they are made up of Prokaryotic cells.

So the correct option is
(A) Animalia
Organism in this kingdom are made up of eukaryotic cells and therefore has membrane bounded organelles, nucleus, they are multicellular (many cells attached to one as mentioned in thr observation) and they lack cell walls.

They both determine the exact age of rocks.