The correct answer is A: Improve instruction. Formative evaluation is a continuous, ongoing process used by educators to monitor student learning during the instructional process. Its primary purpose is to provide immediate feedback to both students and teachers, allowing for timely adjustments to teaching methods and learning strategies. This type of evaluation helps identify areas where students might be struggling or where instruction could be more effective, thereby enhancing the overall learning experience and improving educational outcomes. It's about 'forming' or shaping the learning process as it happens, rather than just assessing the final product.
Option B: Assign grades is incorrect because assigning grades is typically a function of summative evaluation, which occurs at the end of an instructional unit or course to measure overall achievement. Formative assessment's goal is improvement, not final judgment. Option C: Rank students is also incorrect; while assessments can sometimes indirectly indicate student performance relative to peers, the explicit goal of formative evaluation is not to create a competitive ranking. Its focus is on individual learning progress and instructional refinement. Finally, option D: Certify completion is incorrect. Certification is a summative function, often associated with demonstrating mastery or fulfilling course requirements, not the ongoing, diagnostic nature of formative assessment aimed at guiding improvement.