Final answer:
The oldest seafloor is generally less than 200 million years old because the oceanic lithosphere is recycled by subduction at trenches, meaning it doesn't exceed this age due to seafloor spreading.
Step-by-step explanation:
The oldest seafloor is less than 200 million years old primarily because of the ongoing process of seafloor spreading. This geological phenomenon involves the creation of new oceanic crust at mid-ocean ridges and the subsequent subduction of older oceanic lithosphere at trenches. The earth's seafloor is effectively renewed every 100 million years due to this cycle. The never-ending movement of tectonic plates means that as new crust is formed, it slowly moves away from the ridge, cools down, and becomes denser over time. Eventually, when this crust, which is now older and denser, reaches a subduction zone, it is forced down into the mantle and recycled into new rock material.
Option b from the question provided is the correct explanation for this phenomenon: All oceanic lithosphere eventually gets subducted beneath the continents by the time it is 200 million years old. The other answers offered contain inaccuracies; the Earth is over 4.5 billion years old, the oceanic floor does not turn into continental crust, and volcanic heat does not remelt the ocean floor after a certain age.
The oldest seafloor is generally less than 200 million years old because oceanic lithosphere is recycled by subduction at trenches, not exceeding this age due to the constant renewal process of seafloor spreading.