To solve this problem, we need to calculate the final weight percent of the solute after the addition of water.

Given:
Volume of water added = 400 cm3
Weight of the 6% aqueous solution = 600 g

Step 1: Calculating the weight of the solute in the original solution
The weight percent is given as 6%, which means that 100 g of the solution contains 6 g of the solute.

Weight of the solute in the original solution = (6/100) * 600 g = 36 g

Step 2: Calculating the total weight of the final solution
The volume of water added is 400 cm3, which is equal to 400 mL. Since 1 mL of water has a mass of 1 g, the weight of the added water is 400 g.

Total weight of the final solution = weight of the original solution + weight of added water
= 600 g + 400 g = 1000 g

Step 3: Calculating the weight percent of the solute in the final solution
The weight of the solute remains the same after adding water (36 g), but the total weight of the solution has increased to 1000 g.

Weight percent of the solute in the final solution = (36/1000) * 100%
= 3.6%

Therefore, the final weight percent of the solute is 3.6%.

Answer: d. 3.6

In summary:
- The weight of the solute in the original solution is calculated using the weight percent.
- The weight of the solute remains the same, but the total weight of the solution increases after adding water.
- The weight percent of the solute in the final solution is calculated using the weight of the solute and the total weight of the solution.