To understand why the correct answer is option 'D', let's break down the problem step by step.

1. pH Scale:
The pH scale is a measure of the acidity or alkalinity of a solution. It ranges from 0 to 14, where 0 is highly acidic, 7 is neutral, and 14 is highly alkaline. Each unit on the pH scale represents a tenfold difference in acidity or alkalinity.

2. pH and Hydrogen Ion Concentration:
pH is calculated based on the concentration of hydrogen ions (H+) in a solution. The lower the pH, the higher the concentration of H+ ions, and the more acidic the solution. The higher the pH, the lower the concentration of H+ ions, and the more alkaline the solution.

3. Dilution and pH:
When a solution is diluted, the concentration of H+ ions decreases. This means that the pH of the solution will increase. Dilution has the effect of reducing the acidity of a solution.

4. Relationship between pH Values:
To determine how many times a solution of pH 3 must be diluted to reach a pH of 6, we need to consider the difference in hydrogen ion concentration between the two solutions.

- pH 3 has a hydrogen ion concentration of 10^(-3) moles per liter (M).
- pH 6 has a hydrogen ion concentration of 10^(-6) M.

To reach a pH of 6 from pH 3, the concentration of H+ ions needs to decrease by a factor of 10^3. This means that the solution needs to be diluted 1000 times.

Therefore, the correct answer is option 'D' - 1000.