Answer :
The test that will be most useful to determine the identity of the single-celled organism is to "determine if it has a nucleus".
DIFFERENCE BETWEEN A PROTIST AND MONERAN:
A protist is a member of the kingdom protista characterized by it's single cells and eukaryotic nature.
On the other hand, a MONERAN is a member of the kingdom MONERA and characterized by it's unicellular and prokaryotic nature.
The major difference between Monera and protista is their prokaryotic and eukaryotic cells respectively. Prokaryotic cells do not have a membrane bound nucleus while an eukaryotic cell does.
Since the major difference between Monerans and protists is their possession of a membrane bound nucleus, the identity of the single-celled organism is to "determine if it has a nucleus".
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