Option 1: This option suggests that the presence of a lion helps in keeping the trees safe from being cut down. However, lions do not have any direct impact on preventing deforestation or felling of trees. Therefore, option 1 is incorrect.

Option 2: This option states that the presence of a lion adds beauty to the forests. While lions are indeed magnificent creatures, their presence does not directly contribute to the aesthetics of the forest. Therefore, option 2 is incorrect.

Option 3: This option correctly explains that the presence of a lion helps in saving the pastures from being overgrazed. Lions are top predators in their ecosystem and help regulate the populations of herbivores. By controlling herbivore populations, lions prevent excessive grazing and damage to the pastures. Therefore, option 3 is the correct answer.

Option 4: This option suggests that lions keep other carnivorous animals away. While lions may defend their territory against other carnivores, their primary role is not to keep other carnivorous animals in check. Therefore, option 4 is incorrect.

In conclusion, the correct answer is option 3 as it accurately explains the importance of lions in saving pastures from overgrazing.