The correct option is
A Careless
The word 'diligent' is used to refer to someone having or showing care and conscientiousness in one's work and duties.
Option A, forgetful is used to refer to someone who is absent minded and is likely not to remember.
Option B, imprudent refers to someone who does not show care for the consequences of an action.
Option C, careless refers to someone who does not giving sufficient attention or thought to avoiding harm or errors.
Option D, confused refers to a person who is unable to think clearly.
Thus, according to the meaning, the word 'careless' is the most opposite in meaning from the word 'diligent'.