The correct answer is option 3, Anabaena.

To understand why Anabaena belongs to the kingdom Monera, let`s first understand what the kingdom Monera represents. The kingdom Monera is a classification of organisms that includes all prokaryotes, which are simple, single-celled organisms that lack a membrane-bound nucleus and other membrane-bound organelles.

Now, let`s look at the other options:

Option 1: Diatom - Diatoms are a type of eukaryotic algae, which means they have a membrane-bound nucleus. Therefore, diatoms do not belong to the kingdom Monera.

Option 2: Euglena - Euglena is also a eukaryotic organism that has a membrane-bound nucleus. Therefore, euglena does not belong to the kingdom Monera.

Option 4: Ulothrix - Ulothrix is a filamentous green algae, which is also eukaryotic and has a membrane-bound nucleus. Hence, it does not fall under the kingdom Monera.

Option 3: Anabaena - Anabaena is a genus of cyanobacteria, which are prokaryotic organisms without a membrane-bound nucleus. Therefore, Anabaena does belong to the kingdom Monera.