Amazon Elastic Compute Cloud (Amazon EC2) is the correct choice because it provides virtual servers you can sign in to and it supports elastic capacity with pricing that can be charged by the second.

Amazon EC2 gives you on-demand, resizable compute instances with full operating system access so you can manage users, install software, and perform OS-level configuration. EC2 supports a wide range of instance types and Auto Scaling so capacity can grow quickly to meet demand and the billing model includes per-second charges for flexible cost control.

Amazon Lightsail is not ideal because it targets simplified, fixed-price bundles and it does not offer the same granularity of instance types or per-second billing that the startup requested.

AWS Lambda is not suitable because it is a serverless compute model that abstracts away servers and it does not provide OS-level access or a persistent VM that you can sign in to.

Amazon Elastic Container Service (Amazon ECS) is focused on running and orchestrating containers and it does not itself provide a VM to sign in to unless you run containers on EC2 instances which adds complexity compared with launching EC2 directly.

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