The correct answer is A. The application can retrieve its public IPv4 address by accessing the instance metadata from the URL http://169.254.169.254/latest/meta-data/. This metadata service includes various details about the instance, such as its public IPv4 address, which can be specifically retrieved from http://169.254.169.254/latest/meta-data/public-ipv4. Option B relates to user data, which typically contains instance-specific scripts and data that can be used during the instance's initialization, and not its IP address. Option C mentions AMI information, which pertains to the Amazon Machine Image used to launch the instance and does not hold the instance's runtime metadata like its public IP. Option D involves checking the operating system configuration, which is not a reliable method to directly obtain the public IPv4 address of an EC2 instance.