First of all you need to understand the underlying thought process from the consulting firm's point of view, before specifically thinking about the answer to this question.
In consulting, you are essentially in a (professional) client service industry. Thus your primary goal is to help clients and make them more successful.
To make your clients more successful, you need to understand their situation and problem they are facing, analyze it and derive a logical actionable solution which the client is feasable to implement.
Once you immersed yourself into this world and thinking, the way to answer this question is rather obvious. Find a situation which resembles the short summary above of what you do as a consultant - in which someone had a problem, which you understood and analyzed, and finally solved (if it's quantifiable, the better).
Hope this helps!
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