Final answer:
Only 2.5 percent of the Earth's water is fresh, with less than 1 percent accessible for use by living things, making the correct answer to the question 'a. 3 percent'. The majority of Earth's water is saline ocean water at over 97 percent, highlighting the scarcity of readily available fresh water.
Step-by-step explanation:
The amount of fresh water on Earth is significantly less than the total volume of water present. Of the entire water supply, only 2.5 percent is classified as fresh water, according to Figure 37.13 and similar statistics in other provided figures. However, a critical point is that less than 1 percent of this fresh water is readily accessible to organisms, with the majority being trapped in glaciers, ice caps, and underground aquifers.
When considering the availability for human use, this tiny fraction becomes even more significant. With over 97 percent of Earth's water being saline and located in oceans, the remaining 3 percent represents the total proportion of fresh water. Out of this, only a small portion is available for drinking, agricultural, and industrial activities, which is why access to clean, fresh water is a widespread and pressing issue.
In summary, the correct answer to the multiple-choice question is a. 3 percent. This is because out of the entire Earth's water, just 3 percent is fresh, with even less being easily accessible for biological and human needs.