What is the most suitable solution for securing multiple subdomains of an online store within a PKI?

Using wildcard certificates is the most suitable solution for securing multiple subdomains within a Public Key Infrastructure (PKI) because they allow a single certificate to be used for all subdomains of a specified domain. For example, a wildcard certificate for "*.example.com" would secure subdomains like "shop.example.com", "blog.example.com", and "support.example.com" without the need for separate certificates for each one. This not only simplifies the management of certificates but also reduces costs associated with obtaining and renewing individual certificates.

In the context of managing online stores with numerous subdomains, this feature is especially valuable for ensuring a consistent level of security across all services while streamlining administrative tasks. Additionally, it helps avoid the potential complications associated with separately managing revocation, expiration, and renewal processes for multiple certificates.

Other methods, while useful in certain contexts, do not provide the same level of comprehensive and streamlined security for multiple subdomains that wildcard certificates offer.