Which of the following could replace ‘sufficient’ as used in the passage?

Directions: Read the following passage carefully and select the most suitable option according to the corresponding number in the passage.

All animal life on planet Earth depends on sufficient (1) supply of oxygen for its day-to-day (2) survival. The trees that now just dot (3) our landscapes here and there, once heavily blanketed (4) Earth’s surface, but still are able to provide a large porsion (5) of oxygen supply. Through the process of photosynthesis, trees take in (6) carbon dioxide from surrounding air and in return create a sizeable amount of oxygen. Human beings, who cut down large number of trees to create land, shelter and energy, however, pose the most significant (7) threat to survival (8) of worldwide population of trees.