Which one of the following living organisms is an autotroph, that is, capable of preparing its own food from simple inorganic substances?

Introduction / Context:
In basic biology, all organisms are broadly divided into autotrophs and heterotrophs based on how they obtain their food. Autotrophs prepare their own food from inorganic raw materials, while heterotrophs depend directly or indirectly on other organisms. This question checks whether you can correctly identify an autotrophic organism from everyday examples such as fish, algae, butterflies, mushrooms, and earthworms.

Given Data / Assumptions:

• The term autotroph refers to organisms that can synthesise their own food, usually by photosynthesis or chemosynthesis.
• The options include common animals and simple plant like organisms: fish, algae, butterfly, mushroom, and earthworm.
• Fish, butterfly, mushroom, and earthworm are familiar examples of organisms that take in ready made organic food.
• Only one option is a classical example of a photosynthetic, food producing organism.

Concept / Approach:
Autotrophs use inorganic substances like carbon dioxide, water, and minerals to produce organic food. Green plants, algae, and some bacteria are typical autotrophs because they contain chlorophyll and carry out photosynthesis, or they perform chemosynthesis using inorganic chemical energy. Animals, fungi, and most microorganisms are heterotrophs, which means they obtain food by consuming other organisms or their products. To answer the question, we must identify which option is a photosynthetic producer rather than a consumer.

Step-by-Step Solution:
Step 1: Recall that autotrophs are producers in a food chain, while heterotrophs are consumers or decomposers.Step 2: Fish are animals that feed on plants or smaller animals, so they are heterotrophs.Step 3: Algae are simple, often microscopic or aquatic plant like organisms containing chlorophyll and performing photosynthesis, so they are autotrophs.Step 4: Butterflies are insects that feed on nectar and other plant products and are therefore heterotrophs.Step 5: Mushrooms are fungi: they obtain nutrients by decomposing organic matter and are heterotrophic.Step 6: Earthworms are animals that feed on decaying organic matter and soil; they are also heterotrophs.Step 7: Since only algae produce their own food, we select Algae as the correct answer.

Verification / Alternative check:
If you think in terms of trophic levels in an ecosystem, producers (first trophic level) include green plants and algae. Fish, butterflies, mushrooms, and earthworms occur at higher trophic levels as consumers or decomposers. Textbooks often list algae as primary producers in ponds and oceans because they synthesise food using sunlight. This independent ecological viewpoint confirms that among the given options, algae alone are autotrophic.

Why Other Options Are Wrong:
Fish are animals that must eat plants, algae, or other animals and therefore cannot be autotrophs.Butterflies obtain energy from nectar and sometimes other liquids, making them heterotrophic consumers.Mushrooms are fungi that absorb dissolved nutrients from decaying organic matter; they do not photosynthesise.Earthworms ingest soil rich in organic material and help in decomposition, again clearly showing heterotrophic nutrition.

Common Pitfalls:
A common mistake is confusing all green looking or plant related things with autotrophs and all non green things with heterotrophs without checking their actual biology. For example, mushrooms are often assumed to be plants, but they are fungi and cannot photosynthesise. Another pitfall is to think that animals like fish might produce some substances in their body and therefore be autotrophic, which is incorrect. Always remember: autotrophs are producers that manufacture organic food from inorganic substances, and algae are classic representatives of this group.

Final Answer:
The correct answer is Algae.