Which action would best support teachers in implementing differentiated instruction?

Providing training on various instructional strategies and techniques is the most effective action to support teachers in implementing differentiated instruction. Differentiated instruction recognizes that students have diverse learning needs, preferences, and interests, and effective training equips teachers with a range of strategies to address these differences.

By offering professional development focused on differentiation, teachers gain insights into how to tailor their teaching methods, adapt materials, and create learning experiences that meet individual students' needs. This training might include how to group students, modify assignments, use formative assessments, and leverage technology to enhance learning, all of which are essential for successful differentiation in the classroom.

The other choices do not directly address the complexities of differentiated instruction. Allowing teachers to choose their professional development could result in varied and potentially ineffective training that doesn't focus on differentiation. Standardized testing for all students does not consider individual learning styles or needs, while evaluating teachers based on standardized test results might encourage a one-size-fits-all approach to instruction, which contradicts the principles of differentiation.