The correct option is C clear
The word 'nebulous' refers to a vague or ill-defined concept.
Option A, useful refers to something very able or competent in a particular area.
Option B, practical refers to being concerned with the actual doing rather than with theory or ideas.
Option C, clear refers to something easy to perceive, understand or interpret.
Option D, fundamental refers to forming a necessary base or core.
Thus, according to the meaning, the word 'clear' is the opposite of the word 'nebulous'.