**Core Concept**
The question is testing the knowledge of sulphonamide drugs, a class of antimicrobials used to treat various bacterial infections. Sulphonamides work by inhibiting the synthesis of folic acid in bacteria, which is essential for their growth and replication.

**Why the Correct Answer is Right**
The longest-acting sulphonamide is Sulfadimidine (also known as Sulphadimidine), which is a bacteriostatic agent with a half-life of approximately 7-9 hours. Its prolonged action is due to its high lipid solubility, which allows it to be stored in tissues and released slowly. This property makes Sulfadimidine effective in treating urinary tract infections, where it can be excreted in the urine and maintain therapeutic concentrations for an extended period.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a valid choice, as it is not a sulphonamide drug.
**Option B:** This option is incorrect because Sulfamethoxazole, while a long-acting sulphonamide, is primarily used in combination with Trimethoprim (as Co-trimoxazole) for its synergistic antibacterial effects.
**Option C:** This option is incorrect because Sulfadoxine is a long-acting sulphonamide but is primarily used in combination with Pyrimethamine (as Fansidar) for treating malaria.

**Clinical Pearl / High-Yield Fact**
When using sulphonamide drugs, it is essential to remember that they can displace bilirubin from albumin, leading to kernicterus in newborns. This is particularly relevant when treating pregnant women or neonates with sulphonamide antibiotics.

**Correct Answer: C. Sulfadoxine.**