Answer:
B (prokaryote)
Explanation:
Life, in general, has been classified into three domains viz: Eukarya, Bacteria and Archaeae. The first (Eukarya) are organisms with a well defined membrane-bound compartment that houses their genetic material (DNA) called NUCLEUS. On the other hand, the other two domains are generally referred to as prokaryotic organisms because they lack this membrane-bound nucleus. Hence, their genetic material is found naked in a region of the cytoplasm called NUCLEOID.
Therefore, in this question, after microscopically observing and evaluating the uncharacterized organism, It showed that its cell lacked a nucleus. This immediately points out that the cell must be PROKARYOTIC CELL.