Fill in the blank with the most appropriate word from the options given.

The teacher repeated the instructions twice _______ the students could understand clearly.

Option D is the correct answer.

Option A: "Since" explains the reason for an action but doesn’t indicate purpose. It doesn’t fit the sentence’s context of repeating instructions to achieve a specific goal.

Option B: "Because" explains a reason, but this sentence is about the purpose of repeating the instructions, not the cause of the action.

Option C: "While" implies simultaneous actions, which isn’t applicable here.

Option D: This is the best choice, as "so that" indicates the purpose of the teacher repeating the instructions: to ensure understanding.

Option E:  "So as" can express purpose but is typically followed by an infinitive verb (e.g., "so as to make"). It’s grammatically incorrect in this sentence.