The correct answer is 'Use a third-party service to obtain DNS records'. This method allows the penetration tester to gather information about the organization's subdomains without interacting with the target's name servers and potentially revealing their intentions. The third party has likely already collected this data, so the activity is less likely to be traced back to the penetration tester when compared to more direct methods such as querying the organization's name servers. Using an outdated DNS tool might not provide current information. Speculative execution of domain names is not a practical method for DNS enumeration, as it wouldn't necessarily provide accurate results since not every permutation of a domain name is valid or in use. Analyzing web traffic requires direct interaction and is an active reconnaissance technique.