Tarsands containing large molecular hydrocarbons similar to crude oil occur in several parts of the world, but the most important and largest known deposits are found mainly in which of the following countries?

Introduction / Context:
Tar sands, also called oil sands, are mixtures of sand, clay, water, and very heavy hydrocarbons. These hydrocarbons can be processed to produce synthetic crude oil, making tar sands an important unconventional energy resource. This question asks you to identify the country that hosts the most important and largest known deposits of tar sands, a common fact in world energy geography.

Given Data / Assumptions:

• Tar sands contain large molecular hydrocarbons similar to crude oil.
• Such deposits are found in limited quantities in several regions.
• We must identify the country with the most important and largest known occurrence.
• Options include the United States of America, Canada, Australia, and Russia.

Concept / Approach:
The most famous and largest tar sand deposits are in Canada, particularly in the province of Alberta, in fields such as the Athabasca oil sands. These deposits contain enormous reserves of bitumen and have been extensively developed with surface mining and in situ extraction technologies. While tar sands exist in the United States and other countries, the Canadian deposits are by far the best known and largest in terms of recoverable resources, which is why Canada is highlighted in most geography and energy resources textbooks.

Step-by-Step Solution:
Step 1: Recall that when you read about oil sands or tar sands in global energy discussions, Canada, and particularly Alberta, is mentioned very frequently. Step 2: Understand that the Athabasca, Peace River, and Cold Lake deposits in Alberta are often cited as some of the largest bitumen resources on the planet. Step 3: Recognise that the United States has some heavy oil and tar sand deposits, but they are smaller in scale and less prominent compared with those in Canada. Step 4: Note that Australia and Russia have conventional oil and gas resources but are not primarily known for massive tar sand fields comparable to Alberta. Step 5: Therefore, the country described as having the most important known occurrence of tar sands is Canada.

Verification / Alternative check:
Energy resource surveys and environmental reports frequently focus on Canadian oil sands when discussing the environmental and economic implications of tar sand development. These reports emphasise the extraordinary size of the Canadian reserves and the technological challenges of extracting and upgrading bitumen there. By contrast, references to tar sands in the United States, Australia, or Russia are far less prominent in global scale discussions, which confirms that Canada is the correct choice for this exam question.

Why Other Options Are Wrong:

• United States of America: Has some heavy oil and bitumen deposits but not the largest and most important tar sand fields compared with Canada.
• Australia: Known more for coal, iron ore, and other minerals; tar sand deposits do not dominate its energy profile.
• Russia: Possesses vast conventional oil and gas reserves, but its tar sands are not highlighted as the major global example in standard references.

Common Pitfalls:
Learners sometimes choose the United States or Russia simply because they are large countries with big energy sectors, without recalling the specific association between Alberta and oil sands. To avoid this, link the term oil sands or tar sands directly with Alberta in Canada when revising energy geography. This mental association will help you quickly pick Canada when similar questions appear in exams.

Final Answer:
The most important and largest known tar sand deposits are found mainly in Canada.