Explanation:

- The word "hazardous" means something that involves danger or risk.

- Option:1, certain

- This means something that is sure to happen or definite, not linked with danger.

- Option:2, secure

- This refers to something that is safe or protected from danger, the opposite of hazardous.

- Option:3, bulky

- This describes something large and unwieldy, unrelated to danger or safety.

- Option:4, risky

- This means involving risk or danger, similar to "hazardous."

Option:4, risky is the correct synonym for hazardous, as both refer to the presence of risk or danger.

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