Ovarian Mature Cystic Teratoma - Tip-of-the-Iceberg Sign

A 26-year-old nulliparous woman presents for routine gynaecological examination and is found to have a mobile, non-tender right adnexal mass. She is asymptomatic with regular menstrual cycles. Serum tumour markers (AFP, beta-HCG) and CA-125 are all normal. Transvaginal ultrasound reveals a 6.5 cm right ovarian lesion. The structure marked **D** in the diagram—an echogenic mural nodule casting dense distal acoustic shadowing that obscures deeper structures—produces the characteristic "tip-of-the-iceberg" sign. Which of the following is the most likely diagnosis?