Final answer:
The best option to complete Sebastian's sentence is 'In exchange for her dreams'.
Step-by-step explanation:
The best option for completing Sebastian's sentence 'By sunset tomorrow, that child could be giving up her very soul' would be B) In exchange for her dreams. This option fits best with the context of the sentence, as it suggests that the child is sacrificing her soul in order to pursue her dreams. The other options do not provide the same level of coherence and logical connection to the rest of the sentence.