Dry ice is:
a) Solid ${ NH }_{ 3 }$
b) Solid ${ SO }_{ 2 }$
c) Solid ${ CO }_{ 2 }$
d) Solid ${ N }_{ 2 }$

Hint: Dry ice is snow white in colour and has a temperature of -78$^{ o }{ C }$. It is called dry ice since at a temperature above -78$^{ o }{ C }$, instead of converting into a liquid, it sublimes into the gaseous form.

Complete step by step answer:
Solid carbon dioxide is commonly called dry ice since it does not melt into a liquid when heated but rather converts directly into a gas (this process is called sublimation). Let us understand how dry ice is manufactured.
First, the gaseous $ { CO }_{ 2 }$ is compressed under high pressure in order to convert it into liquid $ { CO }_{ 2 }$. Thereafter, the liquid $ { CO }_{ 2 }$ is expanded under reduced pressure such that it converts to $ { CO }_{ 2 }$ snow which is then compressed into dry ice blocks using hydraulic press. Some uses of dry ice are given below:

Uses of dry ice:
1. Dry ice has a temperature of -78$^{ o }{ C }$ due to which it is used to refrigerate food and medical supplies.
2. Dry ice finds its use in airline shipping and dry ice pellets are used for industrial cleaning purposes.
 Nitrogen exists as a solid at a temperature below 63K and becomes a gas at a temperature above 77.2K. The melting point for ammonia is -77.7$ ^{ o }{ C }$. The freezing point of ${ SO }_{ 2 }$ is 16.83$^{o}{C}$.

Therefore the correct answer is (c) Solid ${ CO }_{ 2 }$.

Note: Direct contact with dry ice causes freeze burns and frostbite due to the low temperature (-78$^{ o }{ C }$) of dry ice. The duration of the contact determines the severity of these burns.