What could be a sample answer to this question? 
Say, 10 years. 
Domestic oil production will surpass natural gas production in 10 years. 

How does knowing this figure help us evaluate the argument? This information has no impact. The argument is about reducing the country’s reliance on foreign fuel. How the country’s oil production compares with its natural gas production is not relevant. The heating systems are supposed to be converted from oil to natural gas. A comparison of domestic production of oil versus natural gas and a timeline of when oil production might outstrip natural gap production doesn’t help.

Not useful.

Let’s take the two possible answers and check their impact one by one.

i. Yes, the country is one of the top importers of oil.

What impact does this information have on the argument?
None. 
The conclusion is that the country’s ‘reliance on foreign sources for fuel is likely to decline soon.’ So, that is a comparison of future imports with current imports. Even if the country currently imports a lot of oil, are imports likely to decline? 

Say the country imports a very large amount of oil currently – 1 BILLION barrels of oil per year. Is the quantity likely to decline? The answer to that question will depend on what the country is doing to reduce imports (e.g.domestic production, shifting from oil to natural gas, etc), and not what the quantity of imports is.

ii. No, the country is not one of the top importers of oil.
Say the country imports a not-so-large amount of oil currently – 100,000 barrels of oil per year. Again for the reasons I have mentioned above, this quantity also does not tell us about whether imports will decline. 

Not useful.

i. Say, 20%

ii. Say, 80%

Does either of these figures have any impact on the argument?
No. 

Whether the current proportion is low (~20%) or high (~80%), we don’t learn anything about whether fuel imports will decline. 

Not useful.

Had the answer choice been:
Is the domestic production of hydroelectric, solar and nuclear power increasing?

That answer choice would have been helpful. If the domestic production of other energy sources is increasing, then perhaps fuel imports will decline.
If it is not, then perhaps fuel imports will not decline.

This answer choice essentially will help me understand whether the consumption (demand) of oil  is increasing. It is in line with the first gap I mentioned earlier.

i. If the amount of oil used each year (for the mentioned purposes) is increasing, I believe less than I did before in the claim that fuel imports will likely decline soon. 

ii. If the amount of oil used each year is not increasing, my confidence goes up in the argument. Then I believe more in the claim that fuel imports will probably decline.

Keep in mind, we are looking for an answer choice that will be useful to evaluate the argument. So, even if I am not 100% clear about the situation after getting this new information, that’s fine. As long as the information is useful, I’m good. This answer choice is useful. 

According to the passage, the program to convert the systems from oil to natural gas has already been implemented. How far along they are in the execution of the program is immaterial.

Let’s take cases:

i. Yes, at least one system has been converted
What impact does this information have on the argument?
None.
Even if some systems have been converted, I am not any closer to figuring out whether fuel imports will decline.

ii. No, no systems have been converted yet
What impact does this information have on the argument?
None.
The program was recently implemented. So, even if the execution has not started yet, it does not tell me about how successful the program will be.