How many subnets are required for a Multi-AZ RDS instance in AWS?

Answer - B.

The AWS documentation mentions the following on DB Subnet Groups.

Each DB subnet group should have subnets in at least two Availability Zones in a given region.

When creating a DB instance in VPC, you must select a DB subnet group.

Amazon RDS uses that DB subnet group and your preferred Availability Zone to select a subnet and an IP address within that subnet to associate with your DB instance.

If the primary DB instance of a Multi-AZ deployment fails, Amazon RDS can promote the corresponding standby and subsequently create a new standby using an IP address of the subnet in one of the other Availability Zones.

For more information on multiple IP addresses, please visit the below URL:

http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_VPC.WorkingWithRDSInstanceinaVPC.html#USER_VPC.Subnets

For a Multi-AZ RDS instance in AWS, at least two subnets in two different availability zones (AZs) are required to ensure high availability and fault tolerance.

A Database Subnet Group is a collection of subnets in a VPC that are used to create a high availability environment for RDS instances. When you create a Multi-AZ RDS instance, you specify a Database Subnet Group that contains at least two subnets in different AZs. The Multi-AZ deployment will place a primary database instance in one subnet and a standby database instance in another subnet, both in different AZs. This ensures that in the event of an AZ failure, the standby database in the other subnet can take over and maintain the availability of the database instance.

Therefore, the correct answer is B. 2 subnets.