Option A is correct because reserving a static external IP address ensures the IP is routable and visible on the internet, providing a consistent access point without the need for DNS updates in case of server changes, thus avoiding downtime. This is also the most cost-effective solution. Option B is incorrect due to potential delays in DNS propagation and the need for manual intervention, leading to downtime. Option C is unsuitable as internal IPs are not accessible from the public internet. Option D, while feasible, introduces unnecessary complexity and higher costs compared to using Google Cloud DNS.