The main source of slaves in Africa were raids- the slave traders would go into the villages and kidnapped people ( and often killed those that resisted). This was the cheapest way of getting slaves and the slave traders would go for the cheapest way (just manpower to perform the raids). So the correct option is a). Option b) does not make sense - it would not matter where they sell the slaves - they would make profit anyway. option c) seems correct, but the question is asking for "more" slaves - selling goods was how they started obtaining slaves, but to get more slaves, they resorted to kidnapping themselves.
