The correct answer is D: this act abolished the taking of slaves from Africa. Let's see why:
A, B and C are wrong because:
It's important to not mistake slavery by slave trade. To abolish slavery would mean to end it immediately as a form of work that was an important profit source in many parts of the British Empire. This would be a more radical act that would happen only with the Slave Abolition Act 1833.
D is correct because:
It was thought by then that by abolishing trade of people bought in Africa all slavery would naturally end, that's why this act forbids it. This proved wrong: slavery survived, not only because not all nations respected this act (Brazil, for example, would forbid slave trade only in 1830), but also because the slave trade became a domestic trade. Brazil had slaves until 1888 based on trades between its regions, selling and buying sons and daughters of African slaves.