- Concept: Of outstanding significance means something important and precious.
- Option A is right, monumental refers to something important, so it is the right answer.
- Choice B is erroneous since it means 'based on logic and reason.'
- Choice C is inaccurate since it refers to a "fear that something horrible may happen."
- Choice D is inaccurate since it refers to anything that is "clearly visible or understood."
- Hence, option A is the correct answer.