All of the above options are suitable corrections for the comma-splice error. Each option effectively separates the two independent clauses in a grammatically correct way.
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Lesson 11 explains how to access your feedback, so you need to access your graded work as soon as possible. (adds a coordinating conjunction)
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Lesson 11 explains how to access your feedback. Therefore, you need to access your graded work as soon as possible. (uses a period and a transitional word)
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Lesson 11 explains how to access your feedback; therefore, you need to access your graded work as soon as possible. (uses a semicolon and a transitional word)
So, the correct answer is All of the above.