Upon examining samples of water and mud from a nearby river, you come across an unidentified organism. The organism is unicellular and autotrophic. In addition, you can identify a membrane-bound nucleus in the cells. To what kingdom does it belong?

A protist

Explanation:

In the given question, the characteristic of an unidentified organism are given which are:

1. The organism is unicellular and autotrophic which means that it is a producer and belongs to a plant like organism.

2. The membrane bound nucleus means that the organism is an eukaryote.

The organism is thus an unicellular, autotrophic and an eukaryote could be a species of algae which is considered a protist. The protists are the organisms which shows characteristic of the ukaryotes but are not considered true plants and animals.

Thus, a protist is correct answer.