Final answer:
The truth of the Gaia hypothesis is doubted by scientists largely because it cannot be tested, which is essential for acceptance in the scientific community. Therefore, the correct option is A
Step-by-step explanation:
Scientists are skeptical about the truth of the Gaia hypothesis primarily because it cannot be tested. The Gaia hypothesis suggests that the biosphere is its own living organism, regulating itself to achieve a stable state of homeostasis. For instance, when carbon dioxide levels in the atmosphere increase, plants proliferate, absorbing more CO2, which in theory keeps the atmospheric composition stable.
However, the hypothesis has been criticized as too philosophical or mystical, and its lack of testability poses a significant problem for the scientific method, which relies on empirical evidence and the ability to make predictions that can be experimentally verified. Critics argue that in its current form, the Gaia hypothesis does not provide a framework that could lead to testable predictions or falsifiable hypotheses, an essential aspect of scientific theories.
In contrast, other hypotheses about the natural origin of life on Earth are supported by scientific evidence and are more widely accepted because they can be tested and evaluated in line with the scientific method.