Which of these sentences includes an abstract noun in its context

a) the residents ignored the notices sent by the attorney
b) the tattered envelope held the letter from my sister
c) when john and kim used the flashlight, they could read the address
d) improvement of study habits will lead to better grades

So, an abstract noun is something that you can't touch but instead you can imagine and you know what it is. 
A noun is a person, place, or thing. 

A) The residents ignored the notices sent by the attorney.
The noun in this sentence is "residents" "attorney"

B) The tattered envelope held the letter from my sister
The noun in this sentence is "The tattered envelope" "letter" "sister"

C) When John and Kim used the flashlight, they could read the address 
The noun in this sentence is  "John and Kim" "flashlight" "address"

D) Improvement of study habits will lead to better gradesThe abstract noun in this sentence is "study habits"