The domain that contains multicellular photosynthetic organisms is Eukarya.

Here's why:

* Eukarya includes all organisms with complex cells that have membrane-bound organelles like nuclei, mitochondria, and chloroplasts.

* Multicellularity is a key characteristic of plants, which are a major group within Eukarya.

* Photosynthesis, the process of converting light energy into chemical energy, is also a defining feature of plants.

While bacteria can also be photosynthetic, they are single-celled and belong to the domain Bacteria.