An organism is multicellular, has chloroplasts, and is made up of eukaryotic cells. in which domain would a scientist place this organism?

The answer is Eukarya.

There are three domains in the tree of life: Archaea, Bacteria, and Eukarya. Archae and Bacteria are Prokaryotes, unicellular organisms without a nucleus and organelles. Eukarya mostly includes multicellular organisms with a nucleus and organelles.
The mentioned organism is multicellular made up of eukaryotic cells and has organelles (chloroplasts). According to the characteristics of three different domains, the organism could be placed only in the domain Eukarya.