Introduction
In chemistry, acids are classified as strong or weak based on their ability to dissociate or ionize in water. A weak acid only partially dissociates in water, while a strong acid completely dissociates. In this question, we are asked to identify the weak acid among the given options.

Understanding Acid Strength
Acid strength is determined by the stability of the anion formed when the acid dissociates in water. The more stable the anion, the stronger the acid. Strong acids have highly stable anions, while weak acids have less stable anions.

Analysis of the Options
Let's analyze each option to determine the weak acid:

a) Hydrochloric acid: Hydrochloric acid (HCl) is a strong acid. When it dissolves in water, it completely dissociates into H+ and Cl- ions. Therefore, it is not a weak acid.

b) Nitric acid: Nitric acid (HNO3) is also a strong acid. It fully ionizes in water to produce H+ and NO3- ions. Hence, it is not a weak acid.

c) Acetic acid: Acetic acid (CH3COOH) is a weak acid. It only partially dissociates in water. The equilibrium between the undissociated acid and the ions is shifted towards the reactants. This is because the acetate anion (CH3COO-) is relatively unstable compared to other anions formed by strong acids. Therefore, acetic acid is the correct answer.

d) Sulphuric acid: Sulphuric acid (H2SO4) is a strong acid. It undergoes complete ionization in water to form H+ and SO4^2- ions. Thus, it is not a weak acid.

Conclusion
Among the given options, acetic acid (CH3COOH) is a weak acid. It only partially dissociates in water, forming the acetate anion (CH3COO-), which is relatively unstable compared to the anions formed by strong acids.