a) Solid phase pressure forming, liquid phase pressure forming
b) Liquid phase process, vapour phase pressure forming
c) Melt phase process, liquid phase pressure forming
d) Melt phase process , solid phase pressure forming

Answer: d
Explanation: The two dominant means of thermoforming sheet for food packaging containers are melt phase process and solid phase pressure forming. The melt phase process is most applicable to monolayer structures that have relatively high melt strength at thermoforming temperatures, for example, HIPS, PVC and PC. The SPPF process is primarily used to thermoform PP, a crystalline polymer that is difficult to thermoform uniformly in melt phase machines due to the sharp decrease in melt strength (viscosity) at its melting point.