To solve the question, we need to evaluate each statement one by one to determine which are correct.
### Step-by-Step Solution:
1. **Evaluate Statement (A)**: "Each rank or taxon, in fact, represents a unit of classification."
- This statement is **correct**. In biological classification, each rank (like species, genus, family, etc.) is indeed a unit that helps categorize organisms.
2. **Evaluate Statement (B)**: "Every step in taxonomic hierarchy represents a rank or category."
- This statement is also **correct**. The taxonomic hierarchy consists of various ranks (domain, kingdom, phylum, class, order, family, genus, species), and each step in this hierarchy represents a specific rank.
3. **Evaluate Statement (C)**: "A taxonomic category is a part of the overall taxonomic hierarchy."
- This statement is **correct** as well. Taxonomic categories (like genus, species, etc.) are indeed components of the broader taxonomic hierarchy.
4. **Evaluate Statement (D)**: "Autotrophic bacteria are the most abundant organisms on the earth."
- This statement is **partially correct**. While autotrophic bacteria are indeed abundant and play a crucial role in ecosystems, it is debated whether they are the most abundant organisms compared to other forms of life, such as viruses or other microorganisms. Thus, this statement is not universally accepted as correct.
### Conclusion:
The correct statements are:
- (A) Each rank or taxon, in fact, represents a unit of classification.
- (B) Every step in taxonomic hierarchy represents a rank or category.
- (C) A taxonomic category is a part of the overall taxonomic hierarchy.
### Final Answer:
The correct statements are (A), (B), and (C).
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