4) would be your correct answer
Explanation:
Most reagent forms are going to absorb water from the air; they're called "hygroscopic". Water presence can have a drastic impact on the experiment being performed For fact, it increases the reagent's molecular weight, meaning that anything involving a very specific molarity (the amount of molecules in the final solution) will not function properly.
Heating will help to eliminate water, although some chemicals don't react well to heat, so it shouldn't be used for all. A dessicated environment is simply a means to "dry." That allows the reagent with little water in the air to attach with.
Aldehyde is a group of organic compounds with the general form of RCHO. Oxidation of aldehyde can lead to the formation of the carboxylic acids, with the general form of RCO₂H.
With this oxidation, the typical reagents promoting the oxidation are chromic acids, chromate salts, permanganate, etc.