Answer:
The correct answer is option C which is, "The sand is heated intensely".
Step-by-step explanation:
Glass is the allotropic compound of silicon dioxide. For the formation of glass, first of all the sand which contains the silicon dioxide is melted. For this purpose it is heated intensely.
Due to heating at high temperature, it melts. After melting it ifs cooled immediately. In this step molecules become hard without any special pattern formation.
That is why glass is an allotropic compound. Hence correct answer is option C.