A partially dried clay mineral contains 8% water. The original sample contained 12% water and 45% silica. The % of silica in the partially dried sample is: 

When comparing the quantity of a substance in the sample's original and final states to determine its percentage composition, it is necessary to first presume that the sample's entire amount is being requested, especially if it is expressed as a percentage. The quantity of sample remaining after the change in state would then be determined by adding all the values of the components in the original sample and then the values of the sample in the final state. By using the straightforward technique for calculating percentages, we finally arrive at the component's percentage composition.

We will use the assumption that the sample weighs 100g in total in order to answer this issue and make the computations a little bit clearer. Now, the original sample's 12 water and 45 silica content is specified in the query. We may claim that the original sample comprised 12g of water and 45g of silica since we used 100g as the whole sample. We still have 43g of the total sample remaining after adding these numbers together, which results in 57g.

If we now take a look at the dried sample, it contains 8 water, or 8g of water. Therefore, the dried sample's total weight is 43+45+8g, or 96g.
Now, the dried sample's proportion of silica would be

$\frac{S_o}{M}$×100
 

Where M is the total mass of the sample after drying and S_o is the weight of the silica in the original sample.

$\frac{45}{96}\times 100= 46.87$

Which equals around 47. So, option D would be the best response.

Hence, option A is correct.