Answer:
C. What effect does gravity have?
A. What is gravity?
B. How does the force of gravity affect objects?
Explanation:
Option C is not a clear question because it does not tell effect of gravity on what? Thus, this is the least clear question.
Option B is the most clear question as it clearly asks about a certain aspect of gravity its effect on objects.
Hence, the rank of questions from least clear to most clear is: C, A and B.