The correct answer is Capillaries of the periodontal ligament.

Gingival crevicular fluid (GCF) is an inflammatory exudate found in the sulcus or crevice around the teeth. The primary vascular source that contributes to the formation of GCF is the capillary network within the periodontal ligament. These capillaries are strategically located adjacent to the sulcular epithelium, allowing fluid to pass through the junctional epithelium into the gingival crevice.

Key points to understand:
- The periodontal ligament (PDL) is richly vascularized and provides a direct blood supply to the area around the tooth root and gingival sulcus.
- The capillaries of the PDL respond to inflammatory stimuli and increased vascular permeability by allowing plasma and immune cells to exude into the gingival crevice, forming GCF.
- While superficial gingival blood vessels contribute to the vascularity of the gingiva, they are not the primary source of GCF.
- The alveolar bone marrow capillaries are deeper and less involved in fluid exchange at the gingival margin.
- Lymphatic vessels assist in drainage and immune surveillance but are not the source of the fluid itself.

Therefore, the formation of GCF is strongly linked to the capillaries within the periodontal ligament, which directly supply the fluid and mediators into the gingival sulcus, particularly in states of inflammation.

Reference: Orban's Oral Histology, 15th Edition, Chapter: Periodontium, Page 274-275