Answer:
Yeast or archaea
presence/absence of nucleus and membrane-bound organelles.
Step-by-step explanation:
A unicellular organism that is heterotrophic with cell walls that lack peptidoglycan could be a yeast or an archaea.
Archaea, even though could be autotrophic or heterotrophic grows as single cells and their cell walls lack peptidoglycan.
Yeasts represents a group of unicellular fungi. They are also heterotrophic and lacks peptidoglycan in their cells walls (they have chitin instead).
In order to arrive at a more definitive identification, an information about the presence or absence of nucleus and other membrane-bound organelles will suffice.
Archaea do not have nucleus and membrane-bound organelles while yeast cells have both attributes.