Federation is an access management concept that allows users to authenticate once and access multiple resources or services across different domains or organizations. Federation relies on a trusted third party that stores the user's credentials and provides them to the requested resources or services without exposing them. Password complexity is a security measure that requires users to create passwords that meet certain criteria, such as length, character types, and uniqueness. Password complexity can help prevent brute-force attacks, password guessing, and credential stuffing by making passwords harder to crack or guess.

Impersonating involves pretending to be someone else, in this case, the Chief Executive Officer (CEO), to deceive the employee into taking a specific action (buying gift cards). The attacker is leveraging the authority and trust associated with the CEO's position to manipulate the employee.
Whaling: This phishing attack targets high-profile individuals, such as executives.
An attacker is 'posing' and not 'targeting' a CEO.

A load balancer is the most appropriate solution to ensure accessibility of a business-critical application during peak usage times. It distributes incoming network traffic across multiple servers, optimizing resource utilization, maximizing throughput, minimizing response time, and avoiding overload on any single server. This is particularly crucial for a multinational bank's customer-facing application during high-traffic periods.