Explanation: The correct answer is option B, "or".

The sentence implies that studying hard is necessary to pass the test. The conjunction "or" is used to show that the opposite of studying hard, which is not studying at all, will result in failing the test. Therefore, the correct sentence would be "Study hard, or you will fail the test."

Options A, C, and D do not convey the same meaning as the original sentence. Option A, "unless", implies that the speaker will fail the test if they study hard. Option C, "if", suggests that the speaker may pass the test if they study hard, but it does not indicate what will happen if they do not study hard. Option D, "and", does not make sense in the context of the sentence.