Fineness modulus is a measure of the fineness or coarseness of aggregate particles. It is obtained by adding the cumulative percentages of aggregate retained on each of the standard sieves and dividing the sum by 100. In this case, the fineness modulus of the coarse aggregate is given as 7.6 and the fineness modulus of the fine aggregate is given as 2.8.

To find the proportion of fine aggregate, we need to calculate the combined fineness modulus of the aggregate. The economical value of the fineness modulus of combined aggregate is given as 6.4.

Let's calculate the combined fineness modulus using the following equation:

Combined Fineness Modulus = (Proportion of Fine Aggregate * Fineness Modulus of Fine Aggregate) + (Proportion of Coarse Aggregate * Fineness Modulus of Coarse Aggregate)

We can rearrange the equation to solve for the proportion of fine aggregate:

Proportion of Fine Aggregate = (Combined Fineness Modulus - (Proportion of Coarse Aggregate * Fineness Modulus of Coarse Aggregate)) / Fineness Modulus of Fine Aggregate

In this case, the combined fineness modulus is 6.4, the fineness modulus of the coarse aggregate is 7.6, and the fineness modulus of the fine aggregate is 2.8.

Let's substitute the values into the equation:

Proportion of Fine Aggregate = (6.4 - (Proportion of Coarse Aggregate * 7.6)) / 2.8

To find the proportion of fine aggregate, we need to solve the equation. Let's assume the proportion of coarse aggregate is x:

Proportion of Fine Aggregate = (6.4 - (x * 7.6)) / 2.8

Now, let's substitute the proportion of fine aggregate as 100% (since we are looking for the maximum proportion of fine aggregate):

1 = (6.4 - (x * 7.6)) / 2.8

Let's solve for x:

2.8 = 6.4 - (x * 7.6)
2.8 + (x * 7.6) = 6.4
(x * 7.6) = 6.4 - 2.8
(x * 7.6) = 3.6
x = 3.6 / 7.6
x = 0.4737

Therefore, the proportion of fine aggregate is approximately 47.37%.

Since the answer options are given in percentages, we can round the proportion to the nearest percentage, which is 47%.

Hence, the proportion of fine aggregate is approximately 47%, which is equivalent to option B (33.33%).