How to increase sales and revenue of a Client?
I think the question was asked to see how would you approach the problem. In similar questions, there is not a single answer, and the interviewer does not expect a certain answer instead expects a reasonable approach by you. It is like the famous interview question &quot;How many golf balls can you fit into a swimming pool?&quot;. Such a question is asked to see the analytical thinking of the interviewee.
A structure of the answer may be like,

Churn analysis: understand the reason why customers &quot;not coming&quot; anymore, by doing so you will be able to reduce it.
Do a market analysis to understand the future state of the market to adjust the strategy and products according to the market.
Understanding customer segments (This is one of the most important implementations since you can come up with creative ideas after understanding the segments). Then, suggest the relevant products to relevant segments.
Understanding the competitors and their products. What are the differences between their strategy and ours, their products and ours?
Understanding the market size thus keep the predictions in a realistic range and to make solid steps.
Understanding costs and putting forward optimizations to reduce them.
etc.

These will make them sure that you have good knowledge over-analyzing the company, its strategies, products, etc. The above-mentioned solutions will state them you are not just able to increase the products and revenue, but also decrease the costs to maximize the profit. In this way, you can be a consultant to them.