In what software development model does activity progress in a lock-step sequential process where no phase begins until the previous phase is complete?

"Waterfall" is the correct answer for the above question.

Explanation:

The Waterfall model is a model which is the first model recognized by some scientists is used to states some rules to develop the software. It is based on the phases of the SDLC. The SDLC is the collection of phase that is used in software development. The waterfall model moves like water moves. It means any next phase starts if the previous phase is completed and the user can not move into previous steps of the model. This is also asked by the question. Hence the answer is the waterfall model.