It's highly likely that there are different ways to solve the problem. As with a well-designed multiple-choice question, there will likely be several possible answers. It's the business analyst's responsibility to help the business stakeholders choose the best one.
Before you try to decide on the best solution, it's good to make sure you've identified the right problem.
Review the project documentation and the requirements process so far, and be sure you clearly understand the problem statement. Make sure there has been deliberate discussion of why the problem statement exists. Asking "Why?" at least five times will help you ensure that the problem statement is focused on the actual root cause, not on a surface symptom. Validate that the problem statement describes a problem that the business truly wants to solve, should solve, and can solve. And be aware of any other problems that will "go away" if the present problem is solved.
Once you're sure you've focused on the right problem, take time to consider a range of solutions. Here are two proven options for evaluating solutions:
Solution Selection – Group-style: Gather the best, most knowledgeable thinkers on the topic at a single location. Allow time for silent, focused reflection and idea generation. Then collect everyone's ideas visually (for example, on a white board or flip chart) and invite the group to assess the most popular solutions. Lead a structured discussion around the characteristics of each idea—you might consider using a deliberate evaluation process, such as Edward DeBono's Six Thinking Hats method. Let the ideas suggest which solution best addresses the problems, with the greatest benefits and fewest (or most manageable) obstacles. Be sure to remain neutral and methodical, but do allow instinct and intuition to factor in the conversation.
Solution Selection – Figures-based: Reach out to representatives of each project discipline and business area and invite their ideas for solving the problem. Gather detailed information about the suggested solutions, including the estimated effort, duration, and cost of creating the solution, and the expected benefits—particularly any cost savings and increased revenue. Document the expected costs and benefits for each solution, compare them with the expected benefits, and let the numbers decide. Be sure to include contingency factors for high-probability risks or major gaps in understanding.
A number of other approaches can help, but it's ultimately a simple matter of finding the solution that best solves the problem, that solves the right root cause, and that delivers the best return with the lowest cost and risk. That's simply another way of saying, the solution that delivers the best business value.