The most effective and operationally efficient solution is to use CloudFront's native origin failover capability. By configuring an origin group with the Application Load Balancer (ALB) as the primary origin and an Amazon S3 bucket (hosting the custom error page) as the secondary origin, CloudFront can automatically handle the failure. When the primary origin (ALB) returns an HTTP 503 status code, CloudFront will immediately and automatically retry the request with the secondary origin (S3). This serves the custom error page seamlessly to the user for that specific failed request without any DNS propagation delays or custom code management, thus meeting all requirements with the least operational overhead.

A. Route 53 failover is not suitable for intermittent application-level errors. It relies on health checks that may not fail for 503 errors and is not immediate due to DNS TTLs.
B. This option has the same drawbacks as option A (DNS failover is not immediate) and adds the unnecessary complexity and cost of a second CloudFront distribution.
D. Using a CloudFront Function or Lambda@Edge to handle this requires writing, deploying, and maintaining custom code, which represents higher operational overhead than a simple configuration change like setting up an origin group.

1. AWS CloudFront Developer Guide - Optimizing high availability with CloudFront origin failover: This document explicitly states, "You can set up origin failover for scenarios that require high availability. To get started, you create an origin group with two origins: a primary and a secondary... If the primary origin is unavailable, or if it returns a specific HTTP response status code that indicates a failure, CloudFront automatically switches to the secondary origin." This directly supports the mechanism in option C.

2. AWS CloudFront Developer Guide - Origin group status codes: This section details the specific HTTP status codes (including 503 Service Unavailable) that can trigger a failover from the primary to the secondary origin within an origin group.

3. AWS Route 53 Developer Guide - Failover routing: This guide explains that failover routing works by "routing traffic to a resource when the resource is healthy and to a different resource when the first resource is unhealthy." This is based on health checks and DNS propagation, which is not immediate and less suitable for handling transient, per-request HTTP errors than CloudFront's origin failover.

4. AWS CloudFront Developer Guide - Comparing Lambda@Edge and CloudFront Functions: This documentation clarifies the capabilities of different edge compute options. It shows that implementing the logic described in option D would require Lambda@Edge, which is more complex and operationally intensive than using the built-in origin failover feature.