A wildcard certificate is used to secure multiple subdomains of a single domain. This certificate will contain an asterisk (*) in the domain name field, indicating that it can be used for various subdomains. This is the best choice as it saves time and effort when managing certificates for a domain with many subdomains. A self-signed certificate is not signed by a Certificate Authority (CA) and may cause trust issues with clients. A digital signature is a method to ensure the integrity and authenticity of a message but is not specifically used for securing subdomains. A public key is used as part of asymmetric encryption, but on its own, it's not a certificate type and doesn't demonstrate the function described in the question.