Manuals are useful in providing information for:-

Options

A. Identification of plant species
B. Any one taxon
C. Index of plant species in a particular areas
D. Books related to botanical titles

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the usefulness of manuals in providing information related to plants.

2. **Identifying the Options**: The options provided are:
- Identification of plant species
- Taxonomy and indexing of plant species in particular areas
- Books related to botanical titles

3. **Analyzing the Role of Manuals**: Manuals are tools that help in the identification and classification of plant species. They provide detailed descriptions, characteristics, and names of various species found in specific regions.

4. **Clarifying the Purpose of Manuals**:
- Manuals primarily assist in the identification of plant species by providing information on their names and characteristics.
- They do not focus on indexing or taxonomic classification in a broader sense but rather on specific species.

5. **Conclusion**: Based on the analysis, manuals are most useful for the identification of plant species, particularly in recognizing and naming them.

**Final Answer**: Manuals are useful in providing information for the identification of plant species.

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