Final answer:
The word 'tranquil' needs to be transformed into a verb to fit the sentence. By adding the prefix 'trans-' and the suffix '-ize', we get 'tranquilize', which is the correct word to use in this context.
Step-by-step explanation:
The word in brackets should be transformed to fit the sentence appropriately. Considering the context, the word 'tranquil' should be turned into a verb by adding the prefix 'trans-' and the suffix '-ize', making the word 'tranquilize'. Therefore, the correct sentence would be: 'They had to tranquilize the lion before they could catch it.'
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