The correct answer is: colossal.
Environmental pollution is the contamination of the physical and biological components of the Earth/atmosphere system to such an extent that normal environmental processes are adversely affected. Pollution can take the form of chemical substances or energy, such as noise, heat or light. Pollutants, the components of pollution, can be either foreign substances/energies or naturally occurring contaminants. Pollution is often classed as point source or nonpoint source pollution. Air pollution, water pollution, soil contamination, noise pollution and light pollution are some of the different types of environmental pollution.
The earth is at present in great danger of becoming uninhabitable because of colossal environmental pollution which is going on at an incredible rapid pace. Colossal means extremely large or great in size, extent, or amount. It is the most appropriate word to describe the current state of environmental pollution.
The other options are not as appropriate as colossal. Gigantic means very large or great in size, extent, or amount. However, it does not convey the sense of urgency that is present in the question. Inhuman means not characteristic of or fit for a human being. This option is not appropriate because environmental pollution is not something that is done by humans intentionally. Stupendous means extremely impressive or amazing. This option is not appropriate because environmental pollution is not something that is to be admired.
In conclusion, the correct answer to the question is colossal.