Instead of starting from the definition, look at what each option can actually report about a soil sample.
A chemical test gives numbers like available nitrogen or pH, none of that tells you if the soil crumbles into a nice seedbed or turns into hard clods. A biological test gives microbial counts or organic carbon, useful for fertility but silent on soil structure. Just looking at a field and calling it good or bad tilth gives no repeatable number at all.
An aggregate analysis, on the other hand, sieves the soil dry and wet and reports the size and stability of the clumps, and tilth is defined by exactly that: how the soil aggregates, how big the clumps are, and whether they survive wetting. Since only this test measures the property that tilth actually is, it has to be the right choice.
\[ \text{Correct method} = \text{Aggregate analysis} \]