Directions: Fill in the blank with appropriate word from the given options. The lovers were meeting each other secretly, but their _____ affair was soon known to everyone.

Clandestine is correct because it means 'kept secret or done covertly' - which perfectly describes a secret affair between lovers. Cannot is grammatically incorrect. Covert also means secret but is less commonly used for romantic relationships. Unknown as an adjective doesn't fit - you would say 'little-known affair' not 'unknown affair'.