Understanding the Question:
The question asks which property bloom strength, the standard grading measure for gelatin, is directly proportional to.
Key Concept:
Bloom strength reflects how tightly the polypeptide chains of gelatin cross-link into a gel network once cooled, so anything that changes how much chain length is available to cross-link will change the bloom value.
Checking Each Option:
Calling bloom strength "a measure of the strength and stiffness of the gelatin" just restates its own definition and does not name a separate cause.
Density of the dry gelatin powder relates to manufacturing and packing, not to how strong the hydrated gel network becomes.
Viscosity of the gelatin solution reflects flow resistance in the liquid state, which correlates loosely with chain length but is not the defined driver of gel strength.
Molecular weight controls how many cross-linking points longer chains can form once the gelatin sets, and higher molecular weight gelatin consistently tests at higher bloom values.
Final Answer:
Bloom strength rises directly with the molecular weight of the gelatin.