Final answer:
Among the options, decreased ground temperature is least likely to signal an impending volcanic eruption at Long Valley Caldera, as increased heat is a more usual sign of volcanic activity.
Step-by-step explanation:
The option that is least likely to indicate an impending volcanic eruption at Long Valley Caldera is C. Decreased ground temperature. Typically, before a volcanic eruption, indicators such as increased seismic activity (A), changes in gas emissions (B), and surface deformation (D) are expected. Increased ground temperature, not decreased, could indicate rising magma and heating of the ground. Hence, a decrease in ground temperature would be anomalous and not indicative of an imminent eruption.