Answer: Online standalone CA
Explanation: Standalone CA is the utilized for the extranet or external services .It does not persist active directory and does not need to be domain member either. Because of absence of domain there is no need of domain controller. Due to less layering ,standalone CA becomes more reliable.
The security level on the standalone CA is maintained due to the fact that their is no requirement of domain.The hacker or attacker can only attack the CA panel and cannot access data present on domain.Thus, installation of the online standalone CA should be done as it protect the site.