Female captives among the Cherokee faced two alternatives - they could be married or adopted into clans;
if these options had not been available, however, they would be kept as slaves who labored to support their masters.
A. if these options had not been available, however, they would be kept as slaves---' had not been' is third conditional and the verb of the main clause does not gel with it. This is grammatically wrong.
B. if these options were not be available, however, they might be kept as slaves – not clear as to what the phrase were not be available mean. Is it a typo or intentional?
C. if these options were not available, however, they would be kept as slaves -- the correct answer choice
D. if these options would not be available, however, they will be kept as slaves --- We don’t use the modal ‘would’ in an if clause
E. if these options had not been available, however, they could be kept as slaves --- Past perfect conditional(had not been) and the past tense modal ‘could’ do not go together