The correct option is A Clarity
The word 'ambiguity' refers to the quality of being open to more than interpretation.
Option A, rigidity refers to the inability to be changed or adapted.
Option B, clarity refers to the quality of being certain and definite.
Option C, certainty refers to the quality of being reliably true.
Option D, rationality refers to the quality of being able to think sensibly and logically.
Thus, according to the meaning, the word 'clarity' is the opposite of the word 'ambiguity'.