Explanation:

Comprehensive Detailed Explanation
Relational databases (such as Azure SQL Database, SQL Server, or Azure Database for PostgreSQL) are designed for structured data and ACID-compliant transactions.
Let's analyze the options:
Changes to relationships between entities
While relational databases do handle relationships (via primary/foreign keys), this phrase alone does not describe the core usage scenario for "high volume." Geographically distributed writes This is better suited to NoSQL databases (e.g., Azure Cosmos DB) because they are optimized for global distribution and multi-region writes.
Transactional writes
Correct.
Relational databases excel at handling large volumes of transactional operations (INSERT, UPDATE, DELETE) while ensuring consistency, integrity, and rollback capabilities.
This is the primary workload for OLTP (Online Transaction Processing) systems.
Writes that have varying data structures
This is a use case for NoSQL/document databases like MongoDB or Azure Cosmos DB, not relational systems which require structured schema.
The Answer: Transactional writes
Microsoft References
Azure SQL Database - transactional processing
Data store considerations: relational vs non-relational
OLTP vs OLAP