Phylogenetic classification is a method of organizing and categorizing organisms based on their evolutionary relationships. It focuses on determining the evolutionary history and lineage of organisms by analyzing their genetic and morphological characteristics.

Evolutionary relationships (Option C):
- The correct answer is option C, which states that phylogenetic classification considers evolutionary relationships. This means that organisms that share a more recent common ancestor are grouped together, while organisms that share a more distant common ancestor are placed in separate groups. By examining the similarities and differences in DNA sequences, scientists can reconstruct the evolutionary relationships between different organisms and create phylogenetic trees or cladograms.

Morphological characteristics (Option A):
- Morphological characteristics refer to the physical traits and features of organisms. These include aspects such as body shape, size, color, and anatomical structures. Morphological characteristics have traditionally been used in classification systems to group organisms based on their similarities in appearance. While morphological characteristics are important in phylogenetic classification, they are not the sole determining factor. This is because organisms may have similar morphological characteristics due to convergent evolution or other factors, rather than shared ancestry.

Embryotic structure (Option B):
- Embryotic structure refers to the developmental stages and patterns of an organism from fertilization to birth or hatching. Embryotic structure can provide valuable information about the evolutionary relationships between organisms, as similar developmental patterns suggest common ancestry. However, embryotic structure alone is not sufficient for phylogenetic classification, as it does not take into account the broader evolutionary history or genetic relationships between organisms.

Ultrastructure (Option D):
- Ultrastructure refers to the detailed structures and components within cells and tissues that can only be observed using high-resolution imaging techniques, such as electron microscopy. Ultrastructural characteristics, such as the arrangement of organelles within cells or the presence of specific cellular structures, can provide additional insights into the evolutionary relationships between organisms. However, ultrastructure is not the primary focus of phylogenetic classification, as it is more commonly used in fields such as cell biology and anatomy.

In conclusion, phylogenetic classification primarily considers evolutionary relationships (Option C) between organisms, using a combination of genetic and morphological characteristics. While morphological characteristics, embryotic structure, and ultrastructure are important aspects to consider, they are not the sole determining factors in phylogenetic classification.