Final answer:
The easiest method for a penetration tester to obtain a password with only an email and user account is likely through social engineering, as it exploits human vulnerabilities rather than relying on technical skills.
Step-by-step explanation:
When a penetration tester is attempting to gain access to a remote system with a known user account and email address but no password, the easiest method of obtaining a password for the known account would likely be C. Social engineering. Social engineering involves manipulating people into giving up confidential information, such as passwords or banking information. Since traditional hacking methods like man-in-the-middle attacks, reverse engineering, or hash cracking require technical skills and can be complex, social engineering can be more straightforward. It exploits the human element, which is often the weakest link in security systems.