The most effective method for gaining unauthorized access to a file protected with a long complex password is social engineering. Social engineering is a type of attack that exploits the human factor or the psychological weaknesses of the target, such as trust, curiosity, greed, or fear, to manipulate them into revealing sensitive information, such as passwords, or performing malicious actions, such as opening malicious attachments or clicking malicious links. Social engineering can bypass the technical security controls, such as encryption or authentication, and can be more efficient and successful than other methods that rely on brute force or guesswork. Brute force attack, frequency analysis, and dictionary attack are not the most effective methods for gaining unauthorized access to a file protected with a long complex password, as they require a lot of time, resources, and computing power, and they can be thwarted by the use of strong passwords, password policies, or password managers. References: CISSP All-in-One Exam Guide, Eighth Edition, Chapter 6, Security Assessment and Testing, page 813. Official (ISC)2 CISSP CBK Reference, Fifth Edition, Chapter 6, Security Assessment and Testing, page 829.