Oracle Cloud Infrastructure (OCI) best practices for high availability (HA) recommend distributing application resources across all Availability Domains (ADs) within a region. Most OCI regions are composed of three ADs. A capacity reservation is a resource that is specific to a single Availability Domain; it cannot span multiple ADs. Therefore, to meet the requirement for high availability and ensure that compute capacity is guaranteed in each location, you must create a separate capacity reservation for each of the three Availability Domains. This ensures that if one AD becomes unavailable, you have pre-allocated capacity to maintain operations in the other two.

A. One: A single reservation only guarantees capacity in one AD, which represents a single point of failure and does not fulfill the high availability requirement across ADs.

B. Two: Reserving capacity in only two ADs does not fully utilize the resilience offered by a three-AD region and leaves the application vulnerable if capacity is unavailable in the third AD.

D. Four: This is incorrect as OCI regions with multiple ADs are architected with three Availability Domains, not four.

1. Oracle Cloud Infrastructure Documentation, Overview of Capacity Reservations: "When you create a capacity reservation, you select an availability domain and a compartment. The reservation provides capacity for instances that match the shape and availability domain that you specify." This confirms a reservation is scoped to a single AD.

Source: Oracle Cloud Infrastructure Documentation, Compute > Capacity Reservations > Overview of Capacity Reservations.

2. Oracle Cloud Infrastructure Documentation, Regions and Availability Domains: "A region is composed of one or more availability domains... To get the highest level of availability for your cloud applications, deploy the applications across multiple availability domains." This establishes the principle of using multiple ADs for HA.

Source: Oracle Cloud Infrastructure Documentation, Key Concepts and Terminology > Regions and Availability Domains.

3. Oracle Cloud Infrastructure Documentation, Best Practices for Your Cloud Topology: "To ensure that your resources have high availability, we recommend that you distribute your resources across the availability domains. For a region that has three availability domains, you can create a VCN with a subnet in each of the three availability domains." This explicitly recommends using all three ADs for HA.

Source: Oracle Cloud Infrastructure Documentation, Best Practices for Your Cloud Topology > High Availability.