A Business Activity Model (BAM) is a technique for modelling business activities within an organisation or project at a high level of abstraction. It helps to understand what an organisation does, how it does it, who does it, where it does it and why it does it. Therefore, option D is the correct answer, as a BAM would help the business analyst to identify underlying causes of a problem with declining sales. A BAM would help to analyse how different activities within an organisation contribute or relate to sales performance and identify any gaps or issues that may affect sales performance. A BAM would help to compare current and desired situations or problems and identify what changes or improvements are needed to achieve desired situations or problems. Option A is not a correct answer, as customer journey maps are not a technique for identifying underlying causes of a problem with declining sales. Customer journey maps are a technique for visualising customer experiences with an organisation or product across different touchpoints and stages. They help to understand customer needs, expectations and emotions and how they are met or influenced by an organisation or product. Option B is not a correct answer, as scenario analysis is not a technique for identifying underlying causes of a problem with declining sales. Scenario analysis is a technique for exploring how different scenarios or situations could affect outcomes or impacts of an organisation or project. They help to test feasibility and desirability of different options or solutions under various circumstances or assumptions. Option C is not a correct answer, as use case diagram is not a technique for identifying underlying causes of a problem with declining sales. Use case diagram is a technique for modelling functional requirements of a system or product at a high level of abstraction. They help to understand what functions or features are required or provided by a system or product and how they interact with actors or users.