Correct Answer: Lessen

Solution : The correct choice is the fourth option.

Explanation for each option:

1. Show - This doesn't fit the context as it means to display or exhibit, which is not the same as mitigating.

2. Accelerate - This means to increase the speed or rate of something, which is the opposite of mitigating.

3. Divert - While this word can sometimes mean to redirect or distract, it doesn't convey the same meaning as mitigating.

4. Lessen - This is the most appropriate synonym as it means to make something less severe or intense, which is the intended meaning of the sentence.

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So, the correct sentence would be: "The decision of the committee may lessen the growth of the organisation."