Step 1: Identify the problem. Casting a print in a loose substrate like sand is difficult because the weight of the casting material (e.g., dental stone) can destroy the impression. Therefore, the impression must be stabilized or "fixed" first.
Step 2: Evaluate the options as fixatives.
(1) Melted candle wax: This would be too heavy and hot, destroying the delicate details of the print.
(2) Shellac in alcohol spray: This is a standard technique. A light mist of a fixative spray, like shellac dissolved in alcohol, is applied from a distance. The alcohol evaporates quickly, leaving a thin, hard layer of shellac that stabilizes the sand particles without disturbing the print.
(3) Shellac in water spray: Water would likely cause the sand to clump and lose detail. The solvent needs to evaporate quickly, which is why alcohol is used.
(4) Talcum powder: This is sometimes used to provide contrast for photography, but it is not a fixative and would not stabilize the print for casting.
Conclusion: Shellac in alcohol spray is the correct material used as a fixative before casting in sand.