Question

The highest number of species in the world is represented by
A. Fungi
B. Mosses
C. Lichens
D. Algae

Answer 1

These are single-celled or very complex multicellular organisms. Commonly they include microorganisms such as yeast and moulds, as well as the more familiar mushroom. They are saprophytic in nature. They play an important role in food preparation or preservation for humans.

Complete answer:
Fungi represent the highest number of species in the world. Overall fungi have a worldwide distribution, around $1,20,000$ species of fungi have been described by taxonomists. Fungi are eukaryotes and have a complex cellular organisation which are responsible for decomposition and nutrient cycling through the environment. So accordingly fungi can be unicellular, multicellular or dimorphic, (which is when the fungi are unicellular or multicellular depending on environmental conditions ). The kingdom fungi include some of the most important organisms, in terms of ecological and economical roles. Fungi provide numerous drugs such as penicillin and other antibiotics, food like mushrooms, truffles and the bubbles in bread and beer. Fungi also cause a number of plant and animal diseases; in humans, ringworm, athletes foot etc.
Mosses: they are approximately $12,000$ species. These are non – vascular plants in the land plant division.
Lichens: There are about $20,000$ species. Only $6\%$ of the earth's land surface is covered by lichens. In general, a lichen is a composite organism that emerges from algae living among the filaments of fungi in a mutually beneficial symbiotic relationship.
Algae: also algae are a diverse group of aquatic organisms that have the ability to conduct photosynthesis. Those are about $72,500$ algal species worldwide.

Hence the answer is fungi.

Note:
The fungus kingdom encompasses an enormous diversity of taxa with various ecologies, lifecycle and morphologies. They have been estimated at $2.2$ million to $3.8$ million species. They grow in a wide range of habitats, including extreme environments such as deserts or areas with high salt concentration.