The correct answer is: A. Horizontal Scalability
Horizontal scalability refers to an increase in the performance by the addition of more resources along the service layers. This can be done by adding more servers, or by using a load balancer to distribute the load across multiple servers.
Vertical scalability refers to an increase in the performance by increasing the resources of a single server. This can be done by adding more memory, CPU, or storage.
Diagonal scalability is a combination of horizontal and vertical scalability. It can be achieved by adding more servers and increasing the resources of each server.
None of the above is not a valid option.