The correct answer is option 'A': Floral morphology.

**Explanation:**

Carolus Linnaeus, also known as Carl Linnaeus, was a Swedish botanist who is considered the father of modern taxonomy. He developed a system of classification for plants based on their floral morphology, among other characteristics.

Floral morphology refers to the study of the structure, arrangement, and characteristics of flowers. Linnaeus believed that the reproductive organs of plants, particularly flowers, were the most reliable and consistent features for classifying and identifying different plant species.

**Significance of Floral Morphology in Classification:**

1. **Flower Structure:** Linnaeus observed that flowers have distinctive structures, such as the number and arrangement of floral parts (sepals, petals, stamens, and pistils), the presence or absence of various floral organs, and the shape and size of these structures. These characteristics can vary significantly between different plant species.

2. **Reproductive Organs:** Flowers are the reproductive organs of plants, where the process of sexual reproduction takes place. Linnaeus believed that the reproductive organs were essential for understanding the relationships between different plant species and their classification.

3. **Consistency:** Linnaeus found that floral characteristics were more consistent within a species compared to other plant parts. Flowers tend to have a relatively stable structure and function, making them reliable features for classification.

4. **Ease of Observation:** Linnaeus realized that flowers are easily observable and can be preserved for study and comparison. By focusing on floral characteristics, he made it easier for botanists and scientists to identify and classify plants.

5. **Standardized Terminology:** Linnaeus developed a standardized terminology for describing floral characteristics, such as the number of floral parts, their arrangement, and their fusion. This allowed for clear communication and comparison between different botanists and researchers.

By focusing on floral morphology, Linnaeus was able to create a systematic classification system for plants, known as the binomial nomenclature. This system assigned each plant species a unique two-part name (genus and species), based on their shared characteristics, particularly in floral morphology. This system revolutionized the field of botany and laid the foundation for modern plant taxonomy.