Which of the following ovarian tumor is most prone to undergo torsion during pregnancy?

Dermoid cysts (option c) are most prone to undergo torsion during pregnancy. These cysts, also known as mature cystic teratomas, contain various types of tissues such as hair, teeth, and skin, and can become quite large. Their size and weight, combined with their tendency to be mobile, make them more susceptible to twisting around their supporting structures.