Final answer:
Comparing standardized mathematics assessments before and after implementing the new program is the best way to measure its effectiveness, providing clear, quantifiable data on its impact on student learning outcomes.
Step-by-step explanation:
To evaluate the effectiveness of the new mathematics program initiated by the elementary school principal, option B, which involves comparing the performance exhibited by students on standardized mathematics assessments before and after the adoption of the program, would best serve this purpose. This method provides quantifiable data and directly measures the impact of the program on student learning outcomes. Observing changes in test scores will provide clear evidence of whether students' mathematical capabilities have improved since implementing the new program.