The evaluation used to improve the contents during curriculum development is:

Concept: Educational evaluation is classified by WHEN and WHY it happens relative to a program's development cycle. Formative evaluation (drawing on Scriven's formative-summative distinction) is carried out WHILE a curriculum or instructional material is still being designed or developed, specifically to gather feedback and revise the content before it is finalized. Summative evaluation is carried out AFTER a program has been completed and implemented, to judge its overall effectiveness or worth. Diagnostic evaluation, a related evaluation category, is carried out BEFORE instruction begins, to identify learners' existing knowledge, skills, or gaps.

Application: The question asks for the evaluation used to improve the contents DURING curriculum development -- that is, an evaluation embedded in the development process itself, whose findings feed revisions back into the material before it is finalized. This ongoing, revision-driving role during development is the defining function of formative evaluation.

Cross-check:

• Summative evaluation is conducted only after development and implementation are complete, so by definition it cannot feed improvements back into content that is still being developed.

• Diagnostic evaluation is administered before instruction begins, to assess learners' prior knowledge or skill gaps -- it evaluates learners, not the curriculum content itself, and does not occur during content development.

• The named evaluation types already cover the standard classification used for evaluating content during a development cycle, so ruling out all of them is not warranted here.

Therefore, the evaluation used to improve the contents during curriculum development is formative evaluation.