Explanation
The most important actions to take for the minimum viable solution are to dedupe the instance (B) database and import the leads into instance (A), determine the highest-performing assets and rebuild the campaigns relevant to those assets, audit the instance for critical business functions, and rebuild in instance (A). This is because instance (A) is older and likely has more data, assets, and integrations than instance (B), and it would be easier and faster to migrate the smaller instance into the larger one. Deduping the leads and determining the highest-performing assets are essential steps to avoid data quality issues and maintain campaign performance.
Auditing the instance for critical business functions and rebuilding them in instance (A) are necessary steps to ensure business continuity and alignment. The other options are not as feasible or efficient as this one, because they involve creating a new instance or migrating the larger instance into the smaller one, which would require more time, resources, and complexity.
References: https://docs.marketo.com/display/public/DOCS/Instance+Migration+Guide
https://docs.marketo.com/display/public/DOCS/Best+Practices%3A+Data+Management