Explanation:

Spirulina refers to a biomass of cyanobacteria (blue-green algae) that can be consumed by humans and animals. It is classified as an alga, specifically a type of microalga. Option A, an alga, is correct. Option B, a moss, belongs to the division Bryophyta, which are non-vascular plants. Option C, a liverwort, also belongs to Bryophyta and is a simple land plant. Option D, a fungus, belongs to the kingdom Fungi, which are heterotrophic organisms that absorb nutrients from their environment. These are distinct biological classifications from algae.