Would a policy to import the same quantity of oil every year help?
No.

If a country imports a fixed quantity of oil every year, at the time of an international price rise, they would need to pay more for their imports. Then domestic oil prices would also rise. That would not mitigate the negative economic impact.

I have explained this with an example:

Say, a country imports 100 million litres of oil every year.
Say oil-supply suddenly reduces, and the price per litre of oil goes up by $5.
To continue to import the same quantity of oil, the country would need to pay more money.

Now whether they pass on this additional expense to the consumers or pay from their reserves, the economy will be negatively affected.

Such a policy will not be helpful.

Let’s deal with the following variation first:
(B’) Increasing the oil reserves the country maintains

Would such a policy be helpful?

I think so.

Scenario time:

• A country has an annual demand of 100 million litres of oil 
• When there are no disruptions in oil-supply, the country imports excess oil and builds its reserves
• Over some time, the country has saved up 120 million litres of oil in its reserves
• At that point, there is a disruption in oil-supply

Now, the country could use its oil reserves to sail through the slump.

Granted, maintaining oil inventory would also be expensive. However, those costs would not be sudden. The country can plan to systematically pass on these costs to the consumers.

If the objective is to reduce the long-term economic impact on that nation of sharp and unexpected increases in international oil prices, the policy would be helpful.

Now, back to the original answer choice.

Does ‘increasing the number of oil tankers’ imply that the country is increasing its reserves?

No.

1. Oil tankers may be used only for transportation and not for storage.
2. Even if the oil tankers are used for storage, we don’t know whether the current oil reserves are constrained by the current number of tankers. Maybe the country already has spare tankers.

3. Even if the country currently doesn’t have spare tankers and they increase the number of tankers, we don’t know whether the country will have the money to buy excess oil to keep as reserves.

So a policy to increase the number of oil tankers will not help the country mitigate the economic impact of sudden and sharp oil price increases.

How would such a policy help? 

I think improving diplomatic relations with major oil-producing nations could help. The nations control the bulk of the supply. So, better relations with them might fetch the country a better price even in the case of an oil-supply disruption. 

Suspending the relations could make matters worse.

This option talks about reducing demand. If there is a policy to reduce the demand of oil, even if the supply of oil is disrupted, the economic impact of the sudden increase in oil prices will be mitigated. 

One way to counter the impact on price of reduced supply is to reduce the demand. Such a policy will be helpful.

This option talks about reducing domestic supply. The passage mentioned that domestic oil prices in open-market countries will rise, whether the countries import all or none of their oil. So, if there is a disruption in oil-supply, domestic oil prices will rise – irrespective of the domestic supply.

A supply-side policy will not help. A demand-side policy (like one mentioned in option D) can.