Final answer:
Benchmark site performance metrics should be established during the Planning stage of a migration to provide a baseline for post-migration comparison.
Step-by-step explanation:
Determining the stage at which to benchmark site performance metrics during a migration is crucial for ensuring the success of the process. The correct answer to this question is (a) Planning. It is essential to establish performance benchmarks before executing any site changes or migrations. By doing so, you can have a baseline to compare against post-migration to ensure that performance has not degraded. Benchmarking can help you understand the load times, response times, and other critical performance indicators that affect a user's experience. Ensuring these are maintained or improved in the new environment is key to a successful migration.