Explanation:

- A snake in the grass refers to someone who may seem harmless but is deceitful or treacherous.

- It implies danger that isn’t immediately obvious.

- Option 1: Co-operative person

- This describes someone who works well with others.

- It means being helpful and agreeable.

- Option 2: Treacherous person

- This is the correct meaning of the idiom.

- It describes a person who betrays trust or is disloyal.

- Option 3: Sensitive person

- Refers to someone who is easily affected by feelings or emotions.

- Not related to deceit or hidden danger.

- Option 4: Successful person

- Describes someone who achieves goals or attains prosperity.

- Does not imply deception or secrecy.