Answer: C. Were freed by slave owners.
Manumission is the act of being released from slavery. Manumission can be used as a benevolent gesture to thank a servant after many years of service. In order to portray a sense of humane component, slave masters tend to freely release slaves. Generally, slaves are released from captivity after getting to an age where they are perceived to be less useful.
Sometimes, manumission tends to be very non-charitable. This is so when the slave depends, lives, and survives solely on the slave owner’s funds and can therefore not survive in the outside world. A slave owner however can decide to free a slave with no ample reasons. The slave goes to the outside world with no funds or handicraft and sometimes ends up dead in a short while.