I'll comment on all the answers:
a. Native American leaders decided differences would help identify members of their own tribes.
-cultures rarely get decided on by single people: this is not a correct answer.
b. The different cultures were established in Asia and carried into the Americas.
I think that there would have been too much time time from when the people lived in Asia: during this time any differences would change, so this is also not a reason
c. Constant warfare among the different Native American groups led to cultural differences.
- I don't think warfare causes cultural differences. Compare for example former Yugoslavia, the different sides had similar cultures but still fought
d. The cultures developed to suit the local climate and available resources - I think this is the best answer! for example, in Northern America thre was more opportunity for hunting and less for agriculture than in the south.
