By default, which replica sets are used to write data?

In MongoDB, the default behavior is to write data to the primary replica set member. In a replica set, there is one primary node that accepts all write operations, and the data is then replicated to the secondary nodes for high availability and fault tolerance. The primary node is the only member that receives write operations, while the secondary nodes are read-only by default.

Therefore, the correct answer to your question is: By default, write operations are directed to the primary replica set member in MongoDB.