Interviewing 8-10 customers about how banking fits into their lives and their ideal relationship with their bank is the best way to learn about the unique expectations of the company's customers. This method provides qualitative and contextual insights into the customers' needs, goals, pain points, and emotions. References:
https://trailhead.salesforce.com/en/content/learn/modules/salesforce-strategy-designer-certification-prep/conduct

Dot voting is a democratic and quick way to gauge group consensus on the most valued ideas. It narrows down numerous ideas to those most critical to the group, facilitating focused discussions on priorities and driving alignment on strategic opportunities.

The strategy designer should explore diversifying product offerings to increase market share. This means creating new products or services that appeal to different customer segments or needs, and expanding the customer base and revenue potential of the business. Reference: https://trailhead.salesforce.com/en/content/learn/modules/salesforce-strategy-designer-certification-prep/diversify-product-offerings

Explanation
The technique that should be used to frame the design challenge on the right problems is to construct "How might we..." questions. "How might we..." questions are a way to reframe problems into opportunities for innovation. They are open-ended, optimistic, and user-centered, and they help define the scope and direction of the design project. Designing "For an optimal experience..." questions or creating "As a user, I should..." questions may not be effective for framing the design challenge, as they may imply or limit potential solutions.

The question that CK should seek answers to when determining if the business model is viable is what are customers willing to pay? This question helps evaluate the value proposition and revenue streams of the new service, and whether customers perceive the service as worth paying for. Reference: https://trailhead.salesforce.com/en/content/learn/modules/salesforce-strategy-designer-certification-prep/create-a-business-model-canvas