The two proposals a Solution Architect should recommend to track adoption of migrated functionality from Classic to Lightning are:

B. Track Adoption Rates within the Lightning Usage, and monitor a change in metrics within existing reports and dashboards. C. Provide the CIO a list of the User Stories around the new functionality and the Gap Analysis donebetween Classic and Lightning.

Tracking adoption rates and success metrics is one of the best practices for driving Lightning Experience adoption, as it allows UC to measure how well users are using the migrated functionality and identify areas of improvement or feedback. According to 2, monitoring a change in metrics within existing reports and dashboards can also help UC evaluatethe impact of Lightning Experience on their business performance and user satisfaction.

Providing a list of user stories around the new functionality can help UC communicate the benefits and value of Lightning Experience to their users and stakeholders, aswell as align their expectations and goals with the migration project. According to 3, providing a gap analysis done between Classic and Lightning can also helpUC understand what features or customizations are available or not available in Lightning Experience, as well as plan for any necessary changes or enhancements.

https://trailhead.salesforce.com/content/learn/modules/lex_migration_rollout/lex_migration_rollout_enabling