Final answer:
To report on the Account, Opportunities, and Training Process custom objects, creating a Custom Report Type is the best solution. It allows you to define the relationships between up to four objects and is suitable for the given scenario.
Step-by-step explanation:
To create a report using the Account, Opportunities objects and a Training Process custom object that relate to each other, the best solution would be to create a Custom Report Type (CRT). This is because CRTs allow you to define how objects relate to each other, and you can include up to four objects in a single report type, provided they have a relationship defined in Salesforce. With CRT, you can specify the primary object (for example, Accounts) and relate other objects to this primary object as secondary or tertiary objects, like Opportunities and the Training Process custom object.
A Joined Report is used when you want to combine different report types or when objects do not directly relate to each other. A Cross-Object Formula Field is useful for displaying related data on a record but is not the best fit for reporting on multiple objects. Lastly, a Matrix Report is a report format that allows you to group and summarize data by both rows and columns but does not inherently address the challenge of reporting across unrelated objects.
Therefore, to accurately report on the Account, Opportunities, and Training Process custom objects and their interrelations, you would choose to create a Custom Report Type.