Explanation:

- Spurious: This word means something false or not genuine. It's an adjective for things that mimic being real or true.

- Option 1: Illegitimate: Similar to spurious, it means not legitimate or not in line with accepted standards.

- Option 2: Authentic: This is the opposite of spurious. Authentic means genuine or true.

- Option 3: Mysterious: Refers to something difficult to understand or explain, not directly related to spurious.

- Option 4: Dangerous: Describes something able to cause harm or risk, not directly opposite to spurious.