Final answer:
Decreasing the water-to-powder ratio is the modification that will increase the compressive strength of the set gypsum material, as it results in a denser and stronger structure.
Step-by-step explanation:
To increase the compressive strength of set gypsum products, one should consider the water-to-powder ratio used in mixing the material. Option 1) Increasing the water-to-powder ratio will actually decrease the strength, as the excess water creates a weaker structure once set. Option 4) Decreasing the mixing time can also decrease the strength as insufficient mixing can lead to a less homogeneous mixture and therefore weaker structure.
Thus, the correct modification to increase compressive strength would be 2) Decreasing the water-to-powder ratio, which leads to a denser set material with fewer voids and greater strength. Regarding option 3) increasing the mixing time could potentially aid in creating a more uniform mixture but not necessarily increase the strength more than the proper water-to-powder ratio.