Damage the conclusion

The OA posted by you is a bit of a spoiler because it is a good question. Even after reading the answer I took sometime to figure it out.

Basically, we can narrow it down to A and C because the others are, for obvious reasons, out of context.

But actually even A is irrelevant because there can be more motom accidents in town Y even if many of its residents walk or use public transport means.

C points out why the number of accidents in June and July in town Y is more than that in town X. It is because the traffic becomes four time in these months. So for the motor repair company which wants to set up shop, it has to think of the entire 12 months and not base it's decision on a two month data. So this revelation may indicate that at other times the the business may be more in town X rather than town Y. So C is the answer.

A good question!

Town X and Town Y are roughly equal in size and population. A survey was done measuring traffic pattern during the summer months for each of the past five years. The survey found that, on average in Town Y, there were 28 minor accidents in June as well as 28 in July. During the same two months in Town X, the average was only 14 minor accidents each month. Thus, if the Dentco Autobody Repair franchise were looking to open a new shop in one of the two towns, it would be more likely to succeed in Town Y.
Which of the following, if true, would most seriously damage the conclusion drawn in the passage.
A: More people walk to work or use mass transit in Town Y
B: The state is planning to add a thruway exit for Town Y
C: The traffic volume in Town Y quadruples in the summer because it is a beach
D: In June and July six years ago, Town X had double teh amount of accidents that Y
E: There are rumors of mega mall possibly being constructed in Town X .

Premise:

Accidents/month -- Town X -- Town Y
June-------------------14 ----- 28
July-------------------14 ----- 28

Conclusion: Looking after the accidents in Y, Autobody Repair franchise looking to open in Town Y.

To weaken the conclusion, we need to express that traffic in Y is so only in June and July and not in other 10 months of year.
Only option C does that.