According to science,
1. what is the meaning of "pangaea earthquake"?
2. what is the meaning of "usable water"?

1- The first image shows Pangaea Earthquake. 
2- All fresh water is not usable water because 97% of the water is found in the ocean as salt water and if our bodies are to consume too much salt water, we will eventually become too dehydrated and become very ill. About 2% of the water is stored in glaciers, ice caps and snowy mountain ranges. That leaves around 1% of water for our daily needs. 

Not b or c so that leaves you with a or d. a is the better choice.