This is a phenomenon-hypothesis type argument:
Phenomenon - Insurers in Sparva pay for far more treatments after accidents than insurers in other provinces.
Hypothesis/Explanation - Sparva’s less restrictive policy
In this argument, the author assumes Sparva’s less restrictive policy is the only explanation for insurers paying for far more treatments.
To weaken this type of argument, you say that there is an alternate explanation/hypothesis
To strengthen, you do the opposite i.e you block an alternate explanation.
Now, can you find out an alternate explanation? Well, there may be more number of accidents than the average and that is why insurers had to pay for more treatments.
If you block this case, you get choice E as answer.