What is an activity-based process in which each phase in the SDLC is performed sequentially from planning through implementation and maintenance?

A waterfall methodology.

Step-by-step explanation:

The waterfall methodology is an activity-based process in which each phase in the SDLC is performed sequentially from planning through implementation and maintenance.

A traditional waterfall methodology is one of the many ways of software development methodologies.

In this type of methodology, the project is broken down in linear sequential phases, where the next phase is based on the previous phase. The whole software development will be completed as one single project.