This is where the rubber meets the road. Propose solutions, create testable hypotheses, perform a bunch of tests to gain insight. This is typically the prototyping stage where you have to see what elements of your ideas are compatible with reality.
After you determine a workable solution in the What if stage, the mindset of a team tends to latch on to a single solution. But the result of the What if stage is only a hypothesis, which still needs to be tested before committing resources. .
UX and product designers must adopt a questioning and curious mindset. Skilled questioning leads to better outcomes and paradigm changes within organizations.