How does a buffer resist change in ph upon addition of a strong acid?

Any buffer exists in this equilibrium
 
  HA <=> 
In a buffer, there is a large reservoir of both the undissociated acid (HA) and its conjugate base () 

When a strong acid is added, it reacts with the large reservoir of the conjugate base () forming a salt and water. Since this large reservoir of the conjugate base is used, the ph does not alter drastically, but instead resist the pH change. 

The nucleus inside an atom consists of positively-charged  protons  and neutrally-charged  neutrons. a  proton  is a subatomic particle with a positive electric charge. allprotons  are identical.

The correct answer is d