Final answer:
The changing surface areas of the Pacific and Atlantic Oceans are most likely due to natural geological processes involving tectonic plate movements. Global warming and sea level rise also play a role worldwide through thermal expansion and glacier melt, contributing to sea level rise, but they do not specifically account for the changes between the two oceans.
Step-by-step explanation:
The best explanation for the observation that the surface area of the Pacific Ocean is decreasing while the Atlantic Ocean is increasing is b) Natural geological processes causing shifts in ocean sizes. This is primarily due to the movement of tectonic plates which can cause the expansion of ocean basins in the Atlantic due to seafloor spreading along the Mid-Atlantic Ridge, and the contraction of ocean basins in the Pacific as it has many subduction zones where oceanic plates are sinking beneath continental plates.
Sea level rise and global warming are also relevant factors. Thermal expansion, due to the physical properties of water warming, causes the ocean to occupy a larger volume, raising sea levels. The melting of glaciers also contributes to sea level rise. However, these factors are not directly related to the changing surface areas of the specific oceans mentioned in the question but rather contribute to a global phenomenon.
In terms of human-induced climate change, it does affect ocean currents and can alter the distribution of water in the world's oceans. Nonetheless, the specific changes to the surface area of the Pacific and Atlantic Oceans mentioned cannot be solely attributed to this cause. However, increased greenhouse gas emissions and rising ocean temperatures do affect marine ecosystems and contribute to sea level rise as well as ocean acidification, causing harm to marine life and altering marine food webs.