Incremental delivery in Scrum involves delivering usable product pieces iteratively. The Scrum Guide defines an Increment as: 'a concrete stepping stone toward the Product Goal' that is 'usable' and 'meets the Definition of Done.' Option D---deploying functional increments over the project---reflects this, emphasizing cumulative value delivery.

A: Nonfunctional increments contradict the 'usable' requirement.

B: Testing is part of creating a 'Done' Increment, not a post-delivery step.

C: Process improvement is a Retrospective focus, not incremental delivery.

Exact Extract from Scrum Guide: 'Each Increment is additive to all prior Increments and thoroughly verified, ensuring that all Increments work together. In order to provide value, the Increment must be usable.' (Section: 'The Increment')

Thus, D is correct.