Explanation:

- Option:1 - but

- "She was very exhausted, but she didn’t stop working."

- The conjunction "but" signifies a contrast. Here, the contrast is between her exhaustion and her determination to keep working.

- Answer

- Option:2 - unless

- "She was very exhausted, unless she didn’t stop working."

- "Unless" implies a condition that contradicts the statement. This doesn't fit the meaning intended in the sentence.

- Option:3 - although

- "She was very exhausted, although she didn’t stop working."

- "Although" introduces a surprising fact but isn't the best contrast in this context.

- Option:4 - because

- "She was very exhausted, because she didn’t stop working."

- "Because" provides a reason. However, this doesn't appropriately connect the two ideas here.