Final answer:
The best instructional method during the planning stage of writing a persuasive letter is helping students determine appropriate goals for their letters, as it steers the selection of topics and development of their arguments.
Step-by-step explanation:
When planning a unit on writing a persuasive letter, the best instructional method during the planning stage is C. Helping students determine appropriate goals for their letters. This approach encourages students to think about the purpose of their writing and to identify what they want to achieve, which is a critical step in crafting a convincing argument. Moreover, establishing clear goals can guide students in selecting topics with ample evidence to support their argument, a key factor in persuasive writing as it relies on substantial evidence. Students should be provided with strategies to effectively organize their thoughts and evidence through brainstorming and prewriting activities. Encouraging the use of multiple reasoning strategies, such as analogy, cause and effect, or comparison and contrast, can also help in the development of their letters.