Identify the substance to be separated

Glycerol is obtained from spent-lye in the soap industry. We need the most suitable distillation method for isolating glycerol.

Recall the properties of glycerol

• Glycerol is a high boiling liquid.
• Its boiling point is very high (about $290^\circ \mathrm{C}$).
• On strong heating at atmospheric pressure, it may decompose.

Analyze each option

A. Simple distillation

• Used when the liquid is volatile and impurities are non-volatile, or when boiling points differ greatly.
• Since glycerol has a very high boiling point and may decompose on heating, this is not ideal.

B. Fractional distillation

• Used for separating miscible liquids with close boiling points.
• Here the issue is not close boiling points, but rather the high boiling and decomposition tendency of glycerol.
• So this is not suitable.

C. Steam distillation

• Used for substances that are steam volatile and immiscible with water.
• Glycerol is not steam volatile and is highly soluble in water.
• Hence this is not suitable.

D. Distillation under reduced pressure

• Also called vacuum distillation.
• Suitable for liquids with high boiling points that may decompose near their boiling point.
• Lowering pressure reduces the boiling point, allowing glycerol to distill safely.
• Therefore this is the most suitable method.

Conclusion

The correct option is: $\boxed{\text{D: Distillation under reduced pressure}}$

Comparison with stored correct answer

Stored correct answer = D.

My derived answer also = D. Hence, they agree.