Which insurer should Sana notify of her health change at the anniversary of her policies?

Sana should notify Insurer Y of her health change at the anniversary of her policies because it is likely that the terms and conditions of her policy with Insurer Y specify the need for such disclosure in the event of a health change. Insurers often require updates on a policyholder's health status to assess the risk and determine the appropriateness of coverage. If Insurer Y has specific clauses that necessitate this notification, failure to do so could result in complications regarding claims or the validity of the policy.

The other options suggest a need to notify both insurers or just Insurer X, which may not align with the requirements stipulated by the policies. In typical practices, if there is no requirement from a specific insurer for such notification, then the insured is not obliged to disclose changes to that insurer. Thus, focusing on the correct insurer's policy requirements becomes essential in this context.