It does seem like they are asking the same thing but there is one difference. In (b) it says 'What motivations does Las Casas ascribe to Spanish conquerors and settlers?' While in (c) it says 'How do you think the goals of the Spanish shaped their relations with treatment of native peoples?'
So for question b you are to write about how 'Las Casas' motivated the Spanish Conquerors and Settlers.
For question c you are to write about what you think the goals that the Spanish wanted to achieve had changed and shaped the treatment of the native people.
Hope i helped!