1) No Belts Association claims that it is sole

Why not C?

E still gives a chance for accident as atleast sometimes mother will drive fast.

"oftentimes" in (C) is too vague. Statistically monthers must drive slow within a neighborhood, therby reducing the risk of a major accident.

yes E it is.

The official answer here is E. One important clue to get from the passage here is that No Belts Association is looking for a category of people whom to free from having to use seat belts with the purpose of changing the legislature (in the future) to allow all drivers judge on whether they need to wear their seat belts or not. The reader must pick a statement that puts mothers into the safest possible category.

a. This one is tricky, as it mentions other passengers besides mother drivers. Nevertheless, although wearing seat belts makes mothers nervous, there are multiple other reasons for being nervous. To stay on the safe side and not get nervous about seat belts, get used to wearing them at all times - is the main reason why this explanation can be rejected as the main reason for allowing mothers not use seat belts.
b. This explanation provides no data to help ensure that mothers are safe without wearing seat belts. First of all, we do not know how many accidents happen at speeds up to 15 MPH and how many accidents happen above speeds of 15 MPH. Second of all, even though mothers are more cautious, we do not know who gets into accidents more often - business people or mothers.
c. This statement puts all mothers in a safer position, compared to other drivers, as mothers usually drive 'slow enough', but does not help in creating the 'safe without seat belt' group. What about those mothers who do like to accelerate? If there was more data here to prove that mothers never drive as fast as to injure themselves or their passengers, for example, this statement would be more likely a good explanation of why mothers should be let to drive without their seat belts on.
d. Weakens the argument.
e. This statement indicates that mothers do not use highways, which are the most dangerous place on the road and provides some more data to strengthen the argument in the question stem.