The correct answer is option 1, alga. Spirulina is a type of blue-green algae that is gaining popularity as a rich source of various nutrients. Algae are simple, plant-like organisms that lack roots, stems, and leaves. They can be found in various aquatic environments such as ponds, lakes, and oceans.

Spirulina is particularly known for its high protein content, as it contains all the essential amino acids needed by the human body. It is also rich in vitamins, minerals, and antioxidants, making it a superfood with numerous health benefits.

Option 2, mushroom, is incorrect because mushrooms are fungi, not algae. They belong to a different kingdom and have different nutritional properties.

Option 3, fern, is also incorrect because ferns are a type of vascular plant that reproduce using spores. They are not algae and have different characteristics.

Option 4, flowering plant, is incorrect as well because spirulina is not a flowering plant. Flowering plants belong to the division Magnoliophyta and have distinctive reproductive structures called flowers. Spirulina does not have these characteristics.

In conclusion, spirulina is a type of alga, specifically a blue-green algae, that is known for its