Final answer:
The most effective initial step is to identify specific reading comprehension weaknesses through further assessment, thereby allowing tailored instruction to address these areas.
Step-by-step explanation:
Upon receiving standardized test results showing that 72% of students scored below basic in reading comprehension, the most effective step for a fourth-grade teacher to take before implementing instruction is B) Identify specific areas of weakness through further assessment. This approach enables the teacher to understand the particular reading concepts and skills that students struggle with most. After these areas are identified, targeted interventions can be crafted to improve students' reading abilities. This is consistent with the use of practice and assessment tools such as grasp checks, practice problems, and performance tasks to ascertain and enhance students' grasp of reading comprehension.