Coal carbonisation — During the carbonisation of coal, the temperature range over which the coal softens and forms a plastic layer is approximately:

Introduction:
Caking and plastic behavior of coal during heating govern coke quality and by-product recovery. The plastic layer stage is when coal softens, fuses, and then resolidifies, affecting the development of coke structure.

Given Data / Assumptions:

• Thermal treatment of coal under carbonising conditions.
• Focus on the softening/plasticity temperature interval.

Concept / Approach:
As coal is heated, moisture removal precedes softening. Around mid-range temperatures, coal becomes plastic, enabling coalescence of particles; at higher temperatures, it resolidifies to form semicoke and then coke. The plastic range is characteristically in the few hundreds of degrees Celsius, not at the very low or very high ends of the carbonisation temperature spectrum.

Step-by-Step Solution:
Eliminate very low temperatures (100–150 °C) associated with drying.Eliminate very high temperatures (> 550 °C) where resolidification/coking occurs.Select the commonly cited plastic layer window: 350–450 °C.

Verification / Alternative check:
Empirical tests (e.g., Gieseler fluidity, Audibert–Arnu dilatometry) confirm the onset and peak fluidity in this range for many coking coals.

Why Other Options Are Wrong:

• 100–150 °C: drying/initial devolatilisation, not plasticity.
• 550–650 °C and 700–850 °C: largely post-plastic, resolidifying and coke-strength development regimes.
• 250–300 °C: below typical softening onset for coking coals.

Common Pitfalls:
Confusing the plastic range with ignition or with final coking temperatures used in metallurgical coke ovens.

Final Answer:
350 to 450 °C