Step 1: Definition.
Saccharin is an artificial sweetener, several hundred times sweeter than sucrose.
Step 2: Analysis of options.
- (A) Aliphatic hydrocarbon: Incorrect, saccharin is not a hydrocarbon.
- (B) Sweetening agent: Correct, saccharin is widely used as a non-nutritive sweetener.
- (C) Polynuclear compound: Incorrect, it is not polynuclear.
- (D) Sugar: Incorrect, saccharin is not a natural sugar but a synthetic compound.
Step 3: Conclusion.
Hence, the correct answer is Sweetening agent.