Final answer:
The best guideline for implementing a solid, research-based core instructional program for reading is to consistently implement the program across all classrooms and grade levels.
Step-by-step explanation:
The best guideline for implementing a solid, research-based core instructional program for reading is to d) Consistently implement the program across all classrooms and grade levels. This means that every teacher should follow the same program and use the same materials to ensure consistency and maximize the benefits of the program. By implementing the program consistently, teachers can also collaborate and share insights and strategies to improve instruction.