Ans. is c, i.e. Dermoid cystRef. Novak 14/e, p 510; Dutta Obs. 6/e, p 310"Incidence of dermoid cyst increases two times in pregnancy and it becomes the most commonly diagnosed ovarian tumour during pregnancy." --Dutta Obs. 7/e, p 310"A benign cystic teratoma is the most common neoplasm to undergo torsion." --Novak 14/e, p 510Note: Benign cystic teratoma is another name for dermoid cyst.From the above two lines it is clear that dermoid cyst is the most common ovarian tumour to undergo torsion during pregnancy.For more details on dermoid cyst and other ovarian tumours kindly see "Self Assessment and Review Gynaecology" by the same author.