The correct option is
B Sorrow
In order to fill in the blank with an abstract noun, let's understand what an abstract noun is. A noun that can't be seen and touched is called an abstract noun. For example: I am so happy today. Here, 'happy' is an abstract noun as we can't see and touch 'happy'.
Option A - 'Anger' is an emotion which can't be seen and touched. Though it is an abstract noun, we can't use it here. Since the sentence says 'mother's death', a person can't be angry. Hence, option A is incorrect.
Option C - 'Mercy' is a quality of kindness that makes you forgive someone. It is an emotion and can't be seen and touched, hence it is an abstract noun. We can't use 'mercy' here, as the sentence is about death and not pity/forgiveness. Thus option C is incorrect.
Option D - 'Leniency' is an abstract noun which means to not be severe and not given strong punishments and judgement. As the sentence is about expressing something, we can't use 'leniency'. Hence, option D is incorrect.
Option B - 'Sorrow' is an expression and an emotion, which means to express grief. It is an abstract noun and can be used here to express grief on the occasion of death. Hence, option B is the correct answer.
'I expressed my deep sorrow at his mother's death.'