When conducting a Vane Test, the penetration of the vane into the soil is used to measure:

The Vane Test is primarily designed to measure the in-situ shear strength of soft to medium cohesive soils. During the test, the vane is inserted into the soil and rotated, allowing the penetration depth to be assessed. The resistance encountered during this process reflects the soil's ability to withstand shear stress, which corresponds directly to the soil's shear strength at failure. This measurement is critical for understanding how the soil will behave under loading conditions and is particularly useful in geotechnical engineering applications such as foundation design, slope stability analysis, and assessing earthwork suitability.

Each of the other options pertains to different aspects of soil properties and behavior. Soil moisture content relates to the quantity of water within the soil, which can affect its consistency but is not measured by the Vane Test. Consolidation rates refer to how soil gradually compresses under sustained loading, a process typically evaluated through different testing methods, such as oedometer tests. The plasticity index describes the range of moisture content over which soil behaves plastically, determined through Atterberg limits tests, not the Vane Test. Thus, the correct choice is indeed focused on the measurement of resistance at failure during the Vane Test.