Final answer:
The challenge to the cultural differences argument is presented by the fact that even individuals who believe in normative ethical relativism still make moral judgments about the practices of other cultures, indicating a universality of certain moral values. Additionally, there can be a common underlying principle that killing is morally wrong, despite cultural variations.
Step-by-step explanation:
The challenge to the cultural differences argument is presented by observation 4 and observation 5. Observation 4 highlights that even individuals who claim to believe in normative ethical relativism still make moral judgments about the practices of other cultures. They condemn certain acts, indicating that there are certain acts that are universally regarded as morally wrong. Observation 5 argues that while there may be variations among different cultures, there can still be a common underlying principle that killing is morally wrong, even though societies may differ in what they consider as justified killing.