Final answer:
Effective trainers tailor instruction styles, understand learning processes, and establish specific learning objectives to ensure a focused and measurable training effort.
Step-by-step explanation:
Research has demonstrated that effective trainers tailor the style of the instruction to fit the trainee, possess a robust understanding of how people learn, and importantly, establish specific learning objectives. Option D is the correct answer. Establishing specific learning objectives aligns with the research findings indicating that training is most effective when there is clear guidance on what is expected to be learned and achieved during the training process. This is supported by literature that emphasizes the importance of setting goals, such as learning objectives, to direct learners' attention and actions towards the desired outcome, and making training a more focused and measurable effort.