The correct answer is 'Microservices' because this architecture model provides the ability to scale components independently and ensure high availability. Microservices facilitate continuous delivery and deployment practices that are ideal for online transaction systems with variable load patterns. This architecture enables quick updates and robust security measures for each service without affecting the entire system. 'Monolithic' architecture would be challenging to scale and update without downtime, making it less suitable for high-availability systems. 'Serverless' focuses on event-driven execution, which may lead to unpredictable costs for high-traffic systems and may have limitations around compliance control. 'On-premises' is not an architecture model; it is a deployment method that can still use various architectural models, including microservices.