Explanation:

Let's examine the options:

- Option 1: Unless

- Means "except if."

- ?? Correct answer: The plant will die unless you look after it.

- Implies that care is the condition for the plant to live.

- Option 2: Otherwise

- Means "in another situation" or "if not."

- Doesn't properly fit the sentence structure.

- Option 3: If

- Conditional conjunction that starts a conditional clause.

- Makes sense in a different context: "The plant will die if you don’t look after it."

- Option 4: Until

- Means "up to the time of."

- Incorrect as it doesn't fit the meaning context.