A 62-year-old postmenopausal woman from Delhi presents with progressive abdominal distension and mild abdominal pain for 3 months. She reports early satiety and occasional nausea. On examination, there is abdominal distension with shifting dullness. Pelvic ultrasound reveals a large complex cystic and solid mass arising from the right ovary, measuring 12 cm, with internal septations and solid nodules. CA-125 is markedly elevated at 480 U/mL. Staging CT shows ascites and peritoneal involvement. What is the most likely histological diagnosis?