Answer : The correct answer is -
Third option- Plantae.
As per the given information in the question, an organism was identified by the teacher with the characteristics: eukaryotic, multicellular, autotrophic, and a cell wall made of cellulose.
Organisms belonging to Plantae kingdom are eukaryotes (that is contain true nucleus and membrane bound subcellular compartments, called cell organelles).
They have cell wall (primarily composed of cellulose) around their cell membrane.
They are multicellular organisms (as composed of many cells) and considered as autotrophs (synthesise their own food through photosynthesis).
Thus, the teacher is describing Plantae kingdom.