Use the appropriate word to complete the sentence. ______ you start at once,…

Correct word: unless — means "except if" or "if not."

Paraphrase: Unless you start at once, you will be late = If you do not start at once, you will be late.

Why the other choices are not correct:

• Whether: used for alternatives or doubt (e.g., "I wonder whether..."), not to mean "except if."

• Suppose: used to ask someone to imagine or to introduce a hypothetical assumption, not to express a negative conditional.

• If: introduces a positive condition (e.g., "If you start, ..."). It does not carry the "if not" meaning that the sentence requires.