Final answer:
The entire amount of oil from a reservoir is not usually extracted due to a combination of factors, but most notably due to technological limitations. Current extraction methods cannot reach all the oil, and the economic viability of recovering hard-to-extract oil diminishes as reservoirs are depleted.
Step-by-step explanation:
The entire amount of oil in an underground reservoir is usually not extracted, and the primary reasons include a combination of economic, technological, and geological factors. When considering the economics, extracting every last drop of oil might not be cost-effective, especially as the difficulty in extraction increases. Technologically, even with advanced drilling rigs that can operate in deep water and fracturing techniques used to access 'tight oil', there are limitations. Drilling deeper is not always viable because there is a point at which oil buried too deep will be 'cracked' into gas. Moreover, oil does not exist in vast pools but rather permeates porous rock, and as the 'easy oil' has been depleted, more work is required to coax the remaining oil from the ground.
Although environmental regulations do play a role in the way oil is extracted, including requirements for offshore rigs to prevent pollution and ensure safety against environmental hazards such as hurricanes, they are not the primary reason that not all oil is extracted. The geological truth that oil reserves are not simply waiting to be withdrawn at any rate, and the availability of extraction machinery, also limits the amount of oil that can be feasibly and economically recovered from a reservoir. The correct answer to the question among the options provided is d) Some oil is unreachable by current extraction methods.