Explanation:

Certainly! Here's an explanation of the word "insidious" and the options provided:

- Insidious: This word describes something that is deceitful or sneaky in a harmful way.

- Option 1: Sincere: This word means being genuine, straightforward, and honest. It is a direct opposite of deceitful, which makes this the correct antonym for "insidious."

- Option 2: Treacherous: Similar to insidious; it implies betrayal or deceit. It has a negative connotation.

- Option 3: Astute: Means shrewd or clever, typically with a positive connotation, but not directly opposite to insidious.

- Option 4: Stealthy: This implies being secretive or sneaky, similar in character to insidious.

Given this understanding:

- Option 1: Sincere: