D

Step 1: Understand the concept of a dimensional constant. A dimensional constant is a quantity that has a fixed value and does not change with variation in conditions such as temperature and pressure.

Step 2: Analyze the options:
- Option A: Acceleration due to gravity (approximately 9.81 m/s²) is not constant as it varies with altitude and location.
- Option B: Surface tension of water (approximately 0.0728 N/m) can vary with temperature and impurities in the water.
- Option C: Weight of a standard kilogram mass is also variable depending on the acceleration due to gravity at a location.
- Option D: The velocity of light in vacuum (approximately 3 x 10^8 m/s) is a fundamental constant of nature that has the same value under all conditions in vacuum.

Step 3: Determine the correct answer: Since the velocity of light in a vacuum is a fixed value that does not change regardless of circumstance, it is defined as a dimensional constant.

Therefore, the correct answer is D.