The correct option is
A Dishonesty
The word 'probity' refers to the quality of having strong moral principles like honesty and decency.
Option A, dishonesty refers to deceitfulness shown in someone's character or behaviour.
Option B, timidity refers to lack o courage and confidence.
Option C, treachery refers to betrayal of trust.
Option D, insincerity refers to the quality of not expressing genuine feelings.
Thus, according to the meaning, the word 'dishonesty' is the most opposite in meaning from the word 'probity'.