The correct answer is option 1, which states that the ozone layer of the Earth`s atmosphere prevents the entry of ultraviolet (UV) rays. The ozone layer acts as a shield, absorbing most of the sun`s harmful UV radiation and protecting living organisms on Earth.

Option 2, which states that the ozone layer prevents the entry of X-rays, is incorrect. X-rays have much higher energy compared to UV rays, and the ozone layer is not capable of blocking X-rays.

Option 3, which states that the ozone layer maintains the level of oxygen on Earth, is not directly related to the function of the ozone layer. While the ozone layer does play a role in oxygen production and distribution, its main function is the protection against UV radiation.

Option 4, which states that the ozone layer prevents acid rain on Earth, is unrelated to the ozone layer`s function. Acid rain is caused by emissions of pollutants like sulfur dioxide and nitrogen oxide, not UV radiation.

In conclusion, the ozone layer is important for living organisms because it prevents the entry of harmful UV rays, which can cause damage to DNA, contribute to skin cancer, and harm ecosystems.