Final answer:
The correct answer is (c) Older; colder and therefore, more dense. This is because older oceanic plates are cooler and denser than younger plates, leading them to subduct under lighter plates at collision zones.
Step-by-step explanation:
When two oceanic plates collide, subduction occurs where one plate is forced beneath another. The plate that subducts does so because it is generally older, colder, and hence more dense. This increased density is due to the age of the crust making it cooler and thicker compared to younger crust, which is lighter and hotter. Oceanic plates subduct beneath other plates into high pressure and temperature zones within the mantle, eventually melting and recycling their material, thus helping to balance the crust created at rift zones with the crust being destroyed at subduction zones. Notably, subduction zones are often marked by deep ocean trenches such as the Japan trench, and the process of subduction can eventually lead to volcanic activity as the subducted plate melts and forms magma.
Therefore, the correct answer to the student's question is (c) Older; colder and therefore, more dense.