Final answer:
As descending oceanic crust is subducted beneath continental crust, a. its temperature and pressure increase, which can lead to the formation of volcanoes.
Step-by-step explanation:
When descending oceanic crust scrapes past continental crust, it is involved in a process known as subduction. During subduction, the oceanic crust is forced beneath another plate into the upper mantle where it encounters high temperatures and pressures. This increase in temperature and pressure can cause the crust to melt, leading to volcanic activity as the magma ascends through the overriding plate to form volcanoes. Thus, the correct answer to the student's question is (a) its temperature and pressure increase.