Step 1: Recall what "unless" means.
"Unless" works like "if not", so the clause after it already carries the negative sense on its own. Adding another negative word inside that clause creates a double negative, which changes the meaning.
Step 2: Check the tense used in the main clause.
The main clause "You cannot succeed" is in the simple present tense. Grammar rules ask for the "unless" clause to also stay in the simple present tense, not future forms like "shall" or "will".
Step 3: Test each option.
Option A, "you do not work hard", adds a negative on top of the negative meaning "unless" already carries, so it reads wrong. Option B, "you shall not work hard", uses future tense and a negative, so it breaks both rules. Option C, "you will work hard", uses future tense, which does not match the present tense main clause. Option D, "you work hard", is simple present and positive, matching both rules cleanly.
Final Answer:
Only "you work hard" keeps the tense and meaning consistent with "unless". \[ \boxed{\text{Option D: you work hard}} \]