Explanation:

- Insidious refers to something that is subtly harmful, treacherous, or deceitful. It describes actions that are stealthy and harmful, often with a lurking danger.

- Option 1: Treacherous - This is a synonym, not the antonym. Both insidious and treacherous imply deceitfulness and betrayal.

- Option 2: Sincere - Correct Answer. Sincere means being honest and genuine. It is the direct opposite of deceitful or insidious behavior, which lacks honesty and sincerity.

- Option 3: Stealthy - This is also a synonym. Stealthy describes actions that are furtive and surreptitious, often with a negative connotation similar to insidious.

- Option 4: Astute - This means being shrewd or clever in understanding things. It isn't directly related as an antonym to insidious but rather describes someone's ability.