The correct option is
C lack of enthusiasm and energy
'Lethargy' means the feeling of being tired or having no or little energy.
Option A - 'Serenity' is a state that is peaceful and calm. Thus option A is incorrect.
Option B - 'Animosity' is strong dislike, opposition or anger. Thus option B is incorrect.
Option C - 'Lack of concern' means to barely worry about someone or something.
Option D - 'Lack of enthusiasm and energy' means the feeling of having little energy or being unable or unwilling to do anything. Thus option D is the correct answer.
'Lethargy - lack of enthusiasm and energy.'