${\mathrm{NH}}_3$ can be liquified at ordinary temperature without the application of pressure. But ${\mathrm{O}}_2$ cannot, because

The critical temperature of a gas is defined as the maximum temperature at which a gas can be liquefied that is the temperature above which it cannot be liquefied no matter how much high pressure is applied.
The critical temperature of a gas is a measure of the strength of the intermolecular forces of attraction of the gas. Weaker are the intermolecular forces of attraction, more difficult it is to liquify that gas and hence lower is the critical temperature of that gas. As an example, you can consider oxygen and hydrogen. Both these gases have weak intermolecular forces of attraction and hence are difficult to liquify. Consequently, they have a low critical temperature. On the other hand, carbon dioxide has a much higher critical temperature. This is because the intermolecular forces of attraction between ammonia molecules are very high and therefore it can be easily liquefied. 
The critical temperature is parameter telling about the Van der Waal force of attraction. Lower will be Van der Waal force of attraction, then only it would be difficult to liquefy a gas so we can say we are near to critical temperature.
Hence, the correct option is B