**Not a Strong Electrolyte: Acetic Acid**

**Explanation:**

To determine which of the given substances is not a strong electrolyte, we need to understand the concept of electrolytes and their strength.

**Electrolytes:**
Electrolytes are substances that can conduct electricity when dissolved in water or in a molten state. They dissociate into ions, which are electrically charged particles. These ions are responsible for carrying the electric current.

**Strong Electrolytes:**
Strong electrolytes completely dissociate into ions when dissolved in water or in a molten state. This means that they break apart into their constituent ions, allowing for a higher concentration of ions in the solution and a higher conductivity of electricity.

**Weak Electrolytes:**
Weak electrolytes only partially dissociate into ions when dissolved in water. This means that they exist as both ions and molecules in the solution, resulting in a lower concentration of ions and lower conductivity of electricity compared to strong electrolytes.

**Analysis of Given Substances:**

**(a) Sodium Chloride:**
Sodium chloride (NaCl) is a strong electrolyte. It dissociates into sodium ions (Na+) and chloride ions (Cl-) when dissolved in water or in a molten state.

**(b) Copper Sulphate:**
Copper sulphate (CuSO4) is a strong electrolyte. It dissociates into copper ions (Cu2+), sulphate ions (SO42-), and water molecules when dissolved in water.

**(c) Caustic Soda:**
Caustic soda, also known as sodium hydroxide (NaOH), is a strong electrolyte. It dissociates into sodium ions (Na+) and hydroxide ions (OH-) when dissolved in water.

**(d) Acetic Acid:**
Acetic acid (CH3COOH) is not a strong electrolyte. It is a weak electrolyte. When dissolved in water, only a small portion of acetic acid molecules dissociate into acetate ions (CH3COO-) and hydrogen ions (H+). Most of the acetic acid remains undissociated as molecules.

**Conclusion:**
Among the given substances, acetic acid (CH3COOH) is not a strong electrolyte. It is a weak electrolyte because it only partially dissociates into ions when dissolved in water.