The practical purpose of the classification of living organisms is to facilitate the identification of unknown organisms. This is option 'D'.

Classification of living organisms is a systematic process of organizing and categorizing organisms based on their characteristics and relationships. It provides a way to organize the vast diversity of life on Earth, making it easier for scientists and researchers to study and understand the different species.

By categorizing organisms into different groups, classification allows scientists to effectively identify and describe new species they come across. This is particularly important when it comes to identifying unknown organisms. Here's how the classification system helps in the identification process:

1. Taxonomic Hierarchy:
The classification system uses a hierarchical structure, where organisms are grouped into increasingly specific categories. The highest level is the kingdom, followed by phylum, class, order, family, genus, and species. Each level provides more specific information about the organism, making it easier to identify.

2. Morphological Characteristics:
Classification is primarily based on the morphological characteristics of organisms, such as their physical appearance, anatomy, and structure. These characteristics can be used to distinguish between different groups and identify the species to which an unknown organism belongs.

3. Dichotomous Keys:
Dichotomous keys are tools used in the field of taxonomy to identify organisms based on a series of choices between two characteristics. These keys provide step-by-step guidance to help identify an unknown organism by narrowing down the possibilities with each choice. They are based on the characteristics and traits specific to each taxonomic group.

4. Reference Collections:
Classification allows scientists to create and maintain reference collections, which are repositories of identified and classified organisms. These collections serve as a valuable resource for comparison when identifying new or unknown organisms. By comparing the characteristics of the unknown organism with those in the reference collection, scientists can make accurate identifications.

In conclusion, the practical purpose of the classification of living organisms is to facilitate the identification of unknown organisms. Classification provides a systematic framework for organizing and categorizing species, allowing scientists to effectively identify and describe new organisms they encounter.

Classification helps in identification of unknown organisms....Therefore the correct option will be "
D"